need help with this one.

show the function below has stationary points @ x=-1 x=0 x= 2

$\displaystyle y=\frac{x^4}{4}-\frac{x^3}{3}-x^2$

$\displaystyle 12y=3x^4-4x^3-12x^2$

$\displaystyle

12\frac{dy}{dx}=12x^3-12x^2-24x

$

$\displaystyle

\frac{dy}{dx}=x^3-x^2-2x

$

$\displaystyle

\frac{dy}{dx}=0

$

correct so far?