Remember the order you need to do this. PEMDAS (Please Excuse My Dear Aunt Sally) or Parentheses, Exponents, Multiply, Divide, Add, Subtract. So do the parentheses. (6x^2-3*2) and (4x^2-3) then line them up vertically if that helps (it helps me) and subtract
remember also that when you have to subtract that you switch the signs on the bottom

6x^2-6
-4x^2+3

2x^2-3 (here is your answer)