Quick Polynomial Function Question

For the function

$\displaystyle f(x) = x^{3} - x^{2}=4x-3$

I find the derivative to be:

$\displaystyle f'(x) = 3x^{2}-2x+4$

and I need to set the derivative to equal 0 for this question I'm doing, I would guess I need to use the quadratic formula to do this (if this is wrong please let me know)

But when I use the quadratic formula I get this far:

$\displaystyle

x = \frac{2 \pm \sqrt{4-48}}{6}

$

which gives me:

$\displaystyle

x = \frac{2 \pm \sqrt{-44}}{6}

$

Which can't be right since you can't take the sqrt of a negative number....please help!