Well, we can use this first equation. We could actually use this one up here, but this one is simplified a little bit to actually solve for x. So let's do that, and then we'll substitute back into this thing where we've taken the derivative. So we get 20 times x is equal to x times y. y is 15. And just remember, I could have used this equation, but this is just one step further. We've already cross-multiplied. So it's x times y. y is 15.