Substitute Numbers for Letters in an Expression

In algebra, we use a letter to represent an unknown number. 

Sometimes we need to substitute a number for the letter in an expression.

Example 1

Evaluate 2a - b when a = 6 and b = 7.

Answer 1

2a - b means (2 × a) - b 

So this is (2 × 6) - 7

12 - 7 = 5.

Example 2

Work out the value of 5(x - 2) when x = 9.

Answer 2

Substitute x for 9 to get

5(9 - 2) which means 5 × (9 - 2).

Multiply everything inside the bracket with everything outside the bracket!

5 x 9 = 45

5 x -2 = -10

45 - 10 = 35

Algebra can look complicated but it will soon become second-nature to you.

The important thing is to take it one step at a time and write down all the stages.