Exercises 4.2.22a), 4.2.24 were graded out of 4 points each, with 2 points given for free if you had done the other problem.
When you use the mean value theorem, say it ! (i.e. "By the mean value theorem, there is a c such that..."). It is important to say what the variables represent in an equation, so just writing f'(c) (f(b) - f (a)) = (b - a) does not make too much sense if we do not know where c comes from.