1. Warm Up 2x – 10 9 – 3x 12 9 Solve each equation for x. 1. y = x + 3
Simplify each expression.
x = y – 3
3. 2(x – 5)
2x – 10
4. 12 – 3(x + 1)
9 – 3x
Evaluate each expression for the given value of x.
x + 8 for x = 6
6. 3(x – 7) for x =10 12 9

2. Solving Linear Systems by Substitution
Objective
Solve linear equations in two variables by substitution.

3. Sometimes it is difficult to identify the exact solution to a system by graphing. In this case, you can use a method called substitution.
The goal when using substitution is to reduce the system to one equation that has only one variable. Then you can solve this equation by the methods taught in Chapter 2.

4. Solving Systems of Equations by Substitution
Step 2
Step 3
Step 4
Step 5
Solve for one variable in at least one equation, if necessary.
Step 1
Substitute the resulting expression into the other equation.
Solve that equation to get the value of the first variable.
Substitute that value into one of the original equations and solve.
Write the values from steps 3 and 4 as an ordered pair, (x, y), and check.

5. Solving a System of Linear Equations by Substitution
Solve the system by substitution.
y = 3x
y = x – 2
Step 1 y = 3x
Both equations are solved for y.
y = x – 2
Step 2 y = x – 2
3x = x – 2
Substitute 3x for y in the second equation.
Step 3
–x –x
2x = –2
2x = –2
x = –1
Solve for x. Subtract x from both sides and then divide by 2.

6.   Continued Solve the system by substitution.
Write one of the original equations.
Step 4
y = 3x
y = 3(–1)
y = –3
Substitute –1 for x.
Write the solution as an ordered pair.
Step 5
(–1, –3)
Check Substitute (–1, –3) into both equations in the system.
y = 3x
–3 3(–1)
–3 –3 
y = x – 2
–3 –1 – 2
–3 –3 

7. Solving a System of Linear Equations by Substitution
Solve the system by substitution.
y = x + 1
4x + y = 6
The first equation is solved for y.
Step 1 y = x + 1
Step 2 4x + y = 6
4x + (x + 1) = 6
Substitute x + 1 for y in the second equation.
5x + 1 = 6
Simplify. Solve for x.
Step 3
–1 –1
5x = 5
x = 1
5x = 5
Subtract 1 from both sides.
Divide both sides by 5.

8.   Continued Solve the system by substitution.
Write one of the original equations.
Step 4
y = x + 1
y = 1 + 1
y = 2
Substitute 1 for x.
Write the solution as an ordered pair.
Step 5
(1, 2)
Check Substitute (1, 2) into both equations in the system.
y = x + 1
2 2 
4x + y = 6
4(1)
6 6 

9. Solving a System of Linear Equations by Substitution
Solve the system by substitution.
x + 2y = –1
x – y = 5
Step 1 x + 2y = –1
Solve the first equation for x by subtracting 2y from both sides.
−2y −2y
x = –2y – 1
Step 2 x – y = 5
(–2y – 1) – y = 5
Substitute –2y – 1 for x in the second equation.
–3y – 1 = 5
Simplify.

10. Continued
Step 3
–3y – 1 = 5
Solve for y.
+1 +1
–3y = 6
Add 1 to both sides.
–3y = 6
–3 –3
y = –2
Divide both sides by –3.
Step 4
x – y = 5
Write one of the original equations.
x – (–2) = 5
x + 2 = 5
Substitute –2 for y.
–2 –2
x = 3
Subtract 2 from both sides.
Write the solution as an ordered pair.
Step 5 (3, –2)

11. Try This!
Solve the system by substitution.
y = x + 3
y = 2x + 5
Step 1
y = x + 3
y = 2x + 5
Both equations are solved for y.
Step 2
2x + 5 = x + 3
y = x + 3
Substitute 2x + 5 for y in the first equation.
–x – 5 –x – 5
x = –2
Step 3
2x + 5 = x + 3
Solve for x. Subtract x and 5 from both sides.

12. Try This! Continued
Solve the system by substitution.
Write one of the original equations.
Step 4
y = x + 3
y = –2 + 3
y = 1
Substitute –2 for x.
Step 5
(–2, 1)
Write the solution as an ordered pair.

13. Try This!
Solve the system by substitution.
x = 2y – 4
x + 8y = 16
Step 1 x = 2y – 4
The first equation is solved for x.
(2y – 4) + 8y = 16
x + 8y = 16
Step 2
Substitute 2y – 4 for x in the second equation.
Step 3
10y – 4 = 16
Simplify. Then solve for y.
10y = 20
Add 4 to both sides.
10y =
Divide both sides by 10.
y = 2

14. Try This! Continued
Solve the system by substitution.
Step 4
x + 8y = 16
Write one of the original equations.
x + 8(2) = 16
Substitute 2 for y.
x + 16 = 16
Simplify.
x = 0
– 16 –16
Subtract 16 from both sides.
Write the solution as an ordered pair.
Step 5 (0, 2)

15. Try This!
Solve the system by substitution.
2x + y = –4
x + y = –7
Solve the second equation for x by subtracting y from each side.
Step 1 x + y = –7
– y – y
x = –y – 7
2(–y – 7) + y = –4
x = –y – 7
Step 2
Substitute –y – 7 for x in the first equation.
2(–y – 7) + y = –4
Distribute 2.
–2y – 14 + y = –4

16. Try This! Continued
Solve the system by substitution.
Step 3
–2y – 14 + y = –4
Combine like terms.
–y – 14 = –4
–y = 10
Add 14 to each side.
y = –10
Step 4
x + y = –7
Write one of the original equations.
x + (–10) = –7
Substitute –10 for y.
x – 10 = – 7

17. Try This! Continued
Solve the system by substitution.
Step 5
x – 10 = –7
Add 10 to both sides.
x = 3
Step 6
(3, –10)
Write the solution as an ordered pair.

18.  Try This! y = x – 4 Solve . –x + y = 3
Solve the system algebraically. Use the substitution method because the first equation is solved for y.
–x + (x – 4) = 3
Substitute x – 4 for y in the second equation, and solve.
–4 = 3 
False. The equation is a contradiction.
This system has no solution so it is an inconsistent system.

19. Sometimes you substitute an expression for a variable that has a coefficient. When solving for the second variable in this situation, you can use the Distributive Property.

20. When you solve one equation for a variable, you must substitute the value or expression into the other original equation, not the one that had just been solved.
Caution

21. Using the Distributive Property
y + 6x = 11
Solve by substitution.
3x + 2y = –5
Solve the first equation for y by subtracting 6x from each side.
Step 1 y + 6x = 11
– 6x – 6x
y = –6x + 11
3x + 2(–6x + 11) = –5
3x + 2y = –5
Step 2
Substitute –6x + 11 for y in the second equation.
3x + 2(–6x + 11) = –5
Distribute 2 to the expression in parenthesis.

22. Continued
y + 6x = 11
Solve by substitution.
3x + 2y = –5
Step 3
3x + 2(–6x) + 2(11) = –5
Simplify. Solve for x.
3x – 12x + 22 = –5
–9x + 22 = –5
–9x = –27
– 22 –22
Subtract 22 from both sides.
–9x = –27
– –9
Divide both sides by –9.
x = 3

23. Continued
y + 6x = 11
Solve by substitution.
3x + 2y = –5
Write one of the original equations.
Step 4
y + 6x = 11
y + 6(3) = 11
Substitute 3 for x.
y + 18 = 11
Simplify.
–18 –18
y = –7
Subtract 18 from each side.
Step 5
(3, –7)
Write the solution as an ordered pair.

24. Try This!
–2x + y = 8
Solve by substitution.
3x + 2y = 9
Step 1 –2x + y = 8
Solve the first equation for y by adding 2x to each side.
+ 2x x
y = 2x + 8
3x + 2(2x + 8) = 9
3x + 2y = 9
Step 2
Substitute 2x + 8 for y in the second equation.
3x + 2(2x + 8) = 9
Distribute 2 to the expression in parenthesis.

25. Try This! Continued
–2x + y = 8
Solve by substitution.
3x + 2y = 9
Step 3
3x + 2(2x) + 2(8) = 9
Simplify. Solve for x.
3x + 4x = 9
7x = 9
7x = –7
–16 –16
Subtract 16 from both sides.
7x = –7
Divide both sides by 7.
x = –1

26. Try This! Continued
–2x + y = 8
Solve by substitution.
3x + 2y = 9
Write one of the original equations.
Step 4
–2x + y = 8
–2(–1) + y = 8
Substitute –1 for x.
y + 2 = 8
Simplify.
–2 –2
y = 6
Subtract 2 from each side.
Step 5
(–1, 6)
Write the solution as an ordered pair.

27. Lesson Quiz: Part I
Solve each system by substitution. 1. 2. 3.
y = 2x
(–2, –4)
x = 6y – 11
(1, 2)
3x – 2y = –1
–3x + y = –1
x – y = 4