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. So now it looks like we know what x is. We know what dx dt is. We know what y is.