But when you're dealing with things like undefined, it gets a little bit more hand-wavy. I like to just think of this as the rate at which x is changing with respect to y is zero. And so that got us to the same conclusion. Well, for that to be true, x has to be equal to three y squared. And of course, the xy pair has to also satisfy the equation for the curve. So why don't we use both of these constraints, and then we can solve for x and y. And so the easiest thing I can do, I can think of doing, is let's substitute x with three y squared, because they are the same.