If r wasn't a function of time, then area wouldn't be a function of time. So let me write, instead of just writing r, let me make it explicit that it is a function of time. I'll write r of t. So it's r of t, which we're squaring. We want to find the derivative of this with respect to time. And here, we just have to apply the chain rule. We're taking the derivative of something squared with respect to that something. So the derivative of that something squared with respect to the something is going to be 2 times that something to the first power.