f(x) = x^{3}- 2x^{2}- 5x + 6 find critical points of f on the interval [4,8]

I know I need to take the derivative,which is f^{1}= 3x^{2 }- 4x -5. my problem is finding the critical points. I'm not sure what to do with this, because i cant factor it. well at least i don't think i can. Since the derivative is a quadratic function, can I use quadratic formula? Thanks in advance