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

If this looks daunting, just take it one step at a time and write down each of the steps as you go along. If you need to look at this introduction again, just click on the red help button on the screen.

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