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

However, we must remember that 4b means 4 × b and 4b² means 4 × b × b.

Example

Work out the value of the following expression when a = 4 and b = 9

3a² - 7 - 2√ab

Answer

3a² - 7 - 2√ab

= 3 × a × a - 7 - 2 × √a × b

= 3 × 4 × 4 - 7 - 2 × √4 × 9

= 48 - 7 - 2 × √36

= 48 - 7 - 2 × 6

= 48 - 7 - 12

= 29

Phew - there's a lot to do, but take it slowly, one step at a time and you'll get there!

Let's get started!