These right over here are all going to be equivalent. Now how does this right over here help you? Well, think about the area, think about estimating the area as a bunch of little rectangles here. So that's one rectangle, and then another rectangle right over there, and then another rectangle right over there. So what's the area of each of those rectangles? So the width here, that is going to be x, but we can express x as a function of y. So that's the width right over there, and we know that that's going to be 15 over y.