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.
Work out the value of the following expression when a = 4 and b = -5
3a - 7 - 2b
3a - 7- 2b
= (3 x a) - 7- (2 x b)
= (3 x 4) - 7 - (2 x -5)
= (12) - 7 - (-10)
= 12 - 7 + 10
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.