Right now, r is 3 centimeters. We know dr dt right now is 1 centimeter per second. We know this is 1 centimeter per second. So what's da dt going to be equal to? What's going to be equal to, do that same green, 2 pi times 3 times 1 times 1 centimeter per second. And let's make sure we get the units right. So we have a centimeter times a centimeter.