Minimize the time for rowing and walking.

Problem statement:

A person in a row boat at point P is a distance S miles from a straight shore line.
The point A on the shore is directly opposite the boat. The objective is to travel
from point P to point B on the shore a distance D miles from A in a minimum amount of time.
If the person can row at R miles per hour and walk at W miles per hour where should
the person land the boat between A and B?

The accompanying animation uses S = 4 mi, D = 9 mi, R = 2 mph, W = 3 mph.