You need to know the order of operations (PEMDAS)
Parentheses
Exponents
Multiplication AND Division
Addition AND Subtraction
So look at the equation. Are there exponents? No. Are there parentheses? No. What about multiplication and division? Yes, so you do these first.
3×6=18
So now you have 3-18+3
Now you have to do addition and subtraction (Left to Right)
3-18=-15
Now you have -15+3
-15+3=-12
-12 is your answer.