First of all, I just realized I should've posted this in the homework section. And I know the function is cont. on [a,b] and f'(x) exists on (a,b) so f'(c)=(f(b)-f(a))/(b-a). So f'(c)=(sinb-sina)/(b-a). But I'm not sure how f'(c) comes in to play. And I don't know how to interperate the absolute value either.