# Evaluate expressions

A variable is a letter, for example x, y or z, that represents an unspecified number.

$$6+x=12$$

To evaluate an algebraic expression, you have to substitute a number for each variable and perform the arithmetic operations. In the example above, the variable x is equal to 6 since 6 + 6 = 12.

If we know the value of our variables, we can replace the variables with their values and then evaluate the expression.

**Example**

Calculate the following expression for x=3 and z=2

$$6z+4x=\: ?$$

Solution: Replace x with 3 and z with 2 to evaluate the expression.

$$6z+4x=\: ?$$

$$6\cdot {\color{blue} 2}+ 4\cdot {\color{blue} 3}=?$$

$$\: \, 12+12=24$$

**Video lesson**

Evaluate the following expression for x = 2, y = 5 and z = 4

$$4x+(7-z)-6y$$