On the left-hand side, we can just apply the chain rule. Derivative of tangent of y with respect to y is going to be secant squared of y, which is the same thing as 1 over cosine of y squared. I like to write it this way. It keeps it a little bit simpler, at least in my brain. When we're applying the chain rule, it's going to be the derivative of tangent of y with respect to y times the derivative of y with respect to x. On the right-hand side, the derivative of x with respect to x is going to be equal to 1. If we want to solve for the derivative of y with respect to x, we just multiply both sides times the cosine of y squared.