A bicyclist is riding on a path modeled by the function f(x) = 0.04(8x -x^2), where x and f(x) are measured in miles. Find the rate of change of elevation when x = 2.

f(x) = 0.04(8x-x^2)

The solution manual I looking over says the answer is slope of tangent line at x = 2 is rate of change, and equals about 0.16.

I have no idea how this is about 0.16, so if anyone could shed some light it would be greatly appreciated.