Thank you! (Rolle's Theorem)

Hi all,

I just got back from Calculus final and am so elated. I only missed 2 questions and one of them was the Bonus question which is meant to be harder. It introduced the concept of a fixed point (f(a)=a) and asked us to prove that if f'(x) does not equal 1 for every real number x, then there will be no more than one fixed point

It gave us a hint: Apply Rolle's Theorem to g(x)= f(x) - x

I THINK this was how it was laid out but I couldn't get it. Any help is appreciated. And thanks to this forum! Definitely helped me ace this test :)