I can't figure this one out!

"at noon, ship A is 100 km west of ship B. ship A is sailing south at 35 km/hr, and ship B is sailing north at 25 km/hr. how fast is the distance between the two ships changing at 4:00 pm?"

I have a picture drawn with the path of ship A called x and the path of ship B called y, with a line connecting the ends of the two labeled k, but i cannot think of how to relate these variables! please help!