1. Expression:
An expression is a mathematical "phrase" that stands for a single number; for example
(3x + 1) : is an expression whose value is three times the value of x, plus 1, whatever value the variable x might have.
An expression can also be a single number (9) or variable (p), since those have a numerical value.
3x p p p÷ 9

2. keywords-for-mathematical-operations
To write algebraic expressions and equations, assign a variable to represent the unknown number. In the table below, the letter “x” is used to represent the unknown. In translation problems, the words sum, total, difference, product and quotient imply at least two parts – use parentheses when a sum or difference is multiplied. For example, the phrase "the sum of three times a number and five" translates to "3x + 5," while the phrase "three times the sum of a number and five" translates to "3(x + 5)."
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Practice-exercises-expressions

3. Equations
An equation is a statement that says that the value of two mathematical expressions is equal. In simple words, an equation says that two things are equal. It is denoted by the equal to sign ‘=’.
Example of an Equation: 8+2= 12-2 The above equation says that the left side of the equation is equal to the right side. Thus an equation is a statement that states ‘this equals that’.
The most common equations contain one or more variables. If we let x stand for an unknown number, and write the equation
x= 12+5
We know the left side and right side are equal, so
we can see that x must be 12+5 or 17. This is the
only value that x can have that makes the equation a true
statement. We say that x=17 'satisfies' the equation.

4. worksheets/linear_equations
practice quizzes
algebra/linear-equations