This is the chain rule. So we're left with pi times this is equal to the derivative of our area with respect to time. Now, let me rewrite all of this again, just so it cleans up a little bit. So we have the derivative of our area with respect to time is equal to pi times, actually, let me put that 2 out front, is equal to 2 times pi times, I can now switch back to just calling this r. We know that r is a function of t, so I'll just write 2 pi times r times dr dt. Actually, let me make the r in blue. 2 pi r dr dt. Now, what do we know?