the problem is y = 2 + 3x - X^3 and i'm supposed to find out for values of X is it concave down.

when I take the double derivative it comes out to y" = -6x. I set that equal to zero and y" = 0. How do I find out which values of X are concave down? I know the answer is X > 0 but i'm trying to understand WHY. Please help thanks.