Hint : use the mean value theorem.
You cannot hope to prove this unless you are given that f is continuous at c. Otherwise you can have a function that is continuous and differentiable everywhere in the interval apart from c but which takes an arbitrary value at c itself, or you could have a function that looks like tan(x) but with a small section round pi/2 removed. Then there is a huge discontinuity at c, and yet the derivative is the same both sides of c.
So, assuming f is continuous on (a,b), I suggest that you use the mean value theorem, applied to (a,c] and [c,b), respectively, to show that the left and the right derivative of f at c both exist and are both equal to A.
We are told that . So given there exists such that whenever .
Now suppose that and . By the mean value theorem (as hinted by flyingsquirrel ) for some between and . But if and are both within distance from then so is . It follows that .
Next, let in that last inequality. By continuity of at , this gives . Since this holds whenever , and is an arbitrary positive number, it follows that is differentiable at , with derivative .