Once again, just a chain rule. Derivative of something squared with respect to the something times the derivative of the something with respect to time. And we use similar logic right over here when we want to take the derivative of y squared with respect to time. Derivative of y squared with respect to y times the derivative of y with respect to time. Now on the right hand side of this equation, we once again take the derivative with respect to time. So it's the derivative of s squared with respect to s, which is just 2s, times the derivative of s with respect to time. Once again, this is all just an application of the chain rule.