$\displaystyle f(x)=x^3-kx$, where $\displaystyle k$ can be any number.

Find the values of $\displaystyle k$ such that $\displaystyle f$ has no critical numbers, one critical number, and two critical numbers.

If anyone could help me find one of those, I'll see if I can figure out the rest.

I'm assuming so far we set the derivative to $\displaystyle 0$, but I'm not sure exactly what to do.