Hey guys. Can you please help me with this.

A test question states that f'(x)$\displaystyle \leqslant $5 for all values of x, and that f(0) = -3. What is the maximum possible value for f(4)?What in god's name do I do? I understand that it means that the slope of f is never bigger than 5 but that's it. It has to do with applying mean value thereom