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
Division and Multiplication
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)