In algebra, we use a letter to represent an unknown number.

Sometimes we need 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

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).

Multiply everything inside the bracket with everything outside the bracket!

5 x 9 = 45

5 x -2 = -10

45 - 10 = **35**

Algebra can look complicated but it will soon become second-nature to you.

The important thing is to take it one step at a time and write down all the stages.