Nevermind... I found a way to the answer!
I believe the correct way to start is:
A train leaves a station at noon and travels east at the rate 30 mi/hr. At 2 P.M. of the same day a second train leaves the station and travels south at the rate 25 mi/hr. Express the distance d (mi) between the trains as a function of t (hr), the time the second train has been traveling.
Given answer:
I began with:
Am I incorrect?