We know what x is. We know how fast y is changing, how fast x is changing with respect to time. So what we could try to do here is come up with a relationship between x, y, and s, and then differentiate that relationship with respect to time. And it seems like we have pretty much everything we need to solve for this. So what's the relationship between x, y, and s? Well, we know that this is a right triangle. The streets are perpendicular to each other.