So, in order for the MVT to apply, f needs to be continuous on [a,b] and differentiable on (a,b). You can see both functions follow this rule, so the MVT says that f'(c)(b-a) = f(a) - f(b). So i'll do 11 and see if you can do 12. So f'(c)ln(4) = 4, and f'(c) = 4/ln(4).