We have 20 times, let's see, let me subtract 15 dx dt from both sides of this equation. And we get 5 dx dts. 5 dx dts, I just subtracted this from both sides of the equation. This is 15 dx dts, this is 20. So we have 5 dx dts is equal to, this part right over here is negative 600. And this part right over here is negative 200. So it's equal to negative 800 feet per second.