When we substitute a number for a letter, we simply replace the letter with the number.

However, we must remember that 4b means 4 x b.

Example

Work out the value of the following expression when a = 4 and b = -5

3a - 7 - 2b

Answer

3a - 7- 2b

= (3 x a) - 7 - (2 x b)

= (3 x 4) - 7 - (2 x - 5)

= 12 - 7 - -10

= 12 - 7 + 10

= 15