I don't know how to do this.. But f(x) = x + lnx..

WHat's the value of c for which the instantaneous rate of change of f at x=c is the same as the average rate of change of f over [1, 4]?

What do I do? I did the mean value theorem, but I didn't get the correct answer.. Do I take the derivative and then plug in 1 or 4 or what??