Well, you would have to use, "PEMDAS", (Parentheses, Exponents, Multiplication, Division, Addition, & Subtraction), which are the order of operations to go by.
So first, we do the stuff in parentheses, we'll do 1 + 2 which = 3, and the parentheses around it will = to multiplying 3 times the answer of 6 divided by 2.
So since on the order of operations Addition & Subtraction, and Multiplication & Division could go from any order they appear on in the problem, we'll NOW do 6 divided by 2 which equals 3, times the three from above, which will = 9, so your final answer is 9. =)
I hope I helped! =D