The depth, (in mm), of the water runoff down a slope during a steady rain is

The depth, ℎ (in mm), of the water runoff down a slope during a steady rain is a function of the distance, x (in meters), from the top of the slope,16 so ℎ = f(x). We have f'(15) = 0.02.
(a) What are the units of the 15?
(b) What are the units of the 0.02?
(c) About how much difference in runoff depth is there between two points around 15 meters down the slope if one of them is 4 meters farther from the top of the slope than the other?