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? We know what r is. We know that r, at this moment right in time, is 3 centimeters. Right now, r is 3 centimeters.