So we know what this is. And so if we just know what x is, we can solve for dx dt. So what is x right at this moment? 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.