How do i find the points of inflection points for f(x)=-x^4/4 -x^3/3 +3x^2-(11)??

I already found f'(x)=-x^3-x^2+6x

also I found f"(x)=-3x^2-2x+6

I know I use the quadratic formula but I am not sure what the values are for a,b,and c.

If somebody could show me the quadratic formula with the correct values in it, and solved I would appreciate it.