When you are working something out that involves more than one operation, the order in which they are carried out matters.
The agreed order is:
Brackets
Powers or indices
Multiplication and division
Addition and subtraction
This can be remembered using the word BIDMAS.
However, remember that multiplication and division are equally important, so in a calculation just involving multiplication and division, you carry this out from left to right. Similarly with addition and subtraction.
Examples
6 + 9 ÷ 3 = 6 + 3 = 9. divide first
(6 + 9) ÷ 3 = 15 ÷ 3 = 5. brackets first
15 - 4 × 2 = 15 - 8 = 7. multiply first
(15 - 4) × 2 = 11 × 2 = 22. brackets first
4 × 32 = 4 × 9 = 36. power first
(4 × 3)2 = 122 = 144. brackets first
14 - 4 + 5 = 15. equally important, so left to right.