We already know what this blue stuff is. This blue stuff, that same shade of blue, so this blue stuff right over here, this is equal to all of this business. We've already shaded this in. It's equal to all of this business right over here. So if you were to take all of this green area, which is from a to x plus delta x, and subtract out this blue area, which is exactly what we're doing in the numerator, what are you left with? Well, you're going to be left with what color have I not used yet? Maybe I will use this pink color.