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 4b^{2} means 4 × b × b.

**Example**

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

3a^{2} - 7 - 2√b

**Answer**

3a^{2} - 7 - 2√b

= (3 × a^{2}) - 7 - (2 × √b)

= (3 × 4^{2}) - 7 - (2 x √9)

= (3 × 16) - 7 - (2 x 3)

= 48 - 7 - 6

= **35**