At noon, ship A is 100 km west of ship B. Ship A is sailing south at 35 km/h and ship B is sailing north at 5 km/h. How fast is the distance between the ships changing at 6:00 PM?

I understand how to do these types of problems, but for this one I can't figure out the right equation to differentiate. Anybody want to point me in the right direction?

Thanks for the help