In my calculus class we are doing related rates and i was wondering if somebody could explain the following question:
Car 1 travels due north from point O at 40 mph. A half hour later, Car 2 leaves from a point 10 miles due east of O and travels east at 50 mph. At what rate is the distance between the cars increasing two hours after car 2 leaves?

i figured out dy/dt= 40 mph
dx/dt= 50 mph
and we are looking for dz/dt

i'm not sure wehre to go from there.