Step by step
The question
Question: find the value of 5a − 2b when a = 3 and b = −4.
1 Put the numbers in
5a means 5 times a.
So we write a times sign.
We put 3 in place of a and −4 in place of b.
A negative number goes in brackets, so its minus stays with it.
2 Times and divide
Everyone uses the same order, so we all get the same answer.
Times comes before minus.
5 times 3 is 15.
Times comes before minus.
2 has no sign, so it is plus.
Plus times minus is minus.
2 times 4 is 8.
So we get −8.
3 Plus and minus
Taking away a negative is the same as adding.
So 15 − (−8) becomes 15 + 8.
15 plus 8 is 23.
Careful!
Many people take away before they multiply and get −52.
But times and divide come before plus and minus.
The right way: 5 × 3 − 2 × (−4) = 23.
So 5a − 2b is 23 when a = 3 and b = −4.
Let's look back.
First: put the numbers in.
Then: times and divide.
Last: plus and minus.