The distance from point a and point is equal for travels going back and forth. They would just differ in time and speed because of the sea's speed that propels the vessel. The solutions as as follows:
Distance upstream = Distance downstream
99(t + 2) = (99 + 1)(t)
Solving for t,
t = 198 hours
That means that the distance is equal to:
Distance = 99 miles/hour * (198 + 2 hours)
Distance = 19,800 miles