A swimmer on the south shore of a river wishes to swim to a dock due north of his starting point. His maximum swimming speed is 4.0 km/h, and there is a current flowing at 2.5km/h[W] a-In what direction must he set out and continue swimming if he is to go directly towards the dock? b-If the river is 2.0 km wide, how long does it take him to make the crossing