I've written the derivative of y with respect to x as a function of y. What we're really interested in is writing it as a function of x. To do that, we need to express this somehow in terms of the tangent of y. The reason why the tangent of y is interesting is because we already know that tangent of y is equal to x. If we can rewrite this using a little bit of trigonometric identities, then with tangent of y, we can substitute all the tangent of y's with an x. Let's see if we can do that. This seems a little bit tricky.