Substitute Numbers into Algebraic Expressions

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 and b = -5

3a - 7 - 2b

Answer

3a - 7- 2b

= (3 x a) - 7- (2 x b)

= (3 x 4) - 7 - (2 x -5)

= (12) - 7 - (-10)

= 12 - 7 + 10

= 15

It is easy to get muddled with negative and positive signs if we're not careful, so make sure you remember the rules!

For multiplication and division:

One negative and one positive make a negative.

Two negatives make a positive!

When adding or subtracting:

One negative and one positive sign next to each other mean subtract.

Two negative signs next to each other mean add.

Let's have a go at some questions.