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
3a - 7
Answer
3a - 7
= 3 x a - 7 (3a just means 3 x a
= 3 x 4 - 7 (Change the letter into the number you are given)
= 12 - 7 (Remember BODMAS; Multiply first)
= 5