Given $\frac {dF}{dr} = -2GmMr^{-3}$
Suppose that it is known that the Earth attracts an object with a force that decreases at the rate of $2 N/km$ when $r = 20,000km$. How fast does this force change when $r = 10,000km$?
In this problem, do I just plug in the values for $r$?