I have the function $\displaystyle x^4-2*x^2-1$

I have fount the dirivative

$\displaystyle 4x^3-4x$

and all stationary points as the first part of the question asked but I am stuck on the second half it says :

(b) By using the substitution $\displaystyle u = x^2$, find the horizontal intercepts.

For the most part I dont really understand the question and have no idea were to start to solve it, Any ideas ?