To know which operation you would perform first, you need to use the order of operations. To help you remember this order, you may have heard of the acronym PEMDAS, or the phrase "Please Excuse My Dear Aunt Sally."
The letters (or first letters of each word) stand for the order you have to do your operations to get the right answer:
1) P - Parenthesis
Simplify what is inside the parenthesis first
2) E - Exponents
Simplify your exponents (roots and powers)
3) M - Multiplication, D - Division
Multiply or divide from left to right
4) A - Addition, S - Subtraction
Add or subtract from left to right
Using your order of operations, you can see that you should do what is inside the parenthesis first, making subtraction of 5 − 3 your first operation.
------
Answer: Subtraction (5-3)