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. And then we're going to have to make it clear. So this is the derivative of r of t-squared with respect to r of t. The derivative of something squared with respect to that something. If it was the derivative of x squared with respect to x, we'd have 2x.