Substitute Numbers for Letters into an Expression

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

Sometimes we want 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

i.e.  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).

Now work out the brackets first to get 9 - 2 = 7.

So this is 5 × 7 = 35