Show that f(x)=x^4-2x^2 satisfies the hypothesis of Rolle's Theorem on [-1,1]. Then find all numbers c that satisfy the conclusion of Rolle's Theorem.

I believe Rolle's Thm states that f(a) has to equal f(b) to find a number c... I calculated f(a) to be 1 and f(b) to be -1, therefore I can't find c, right? Or did I do something wrong?