a motorboat traveling 5m/s, EAST encounters a current traveling 2.5m/s North, in what direction should a boat travel so to land directlyat the opposite side of the riverbank?

I assume East is straight across.

boat propulsion speed = 5 in direction X degrees south of East
5 sin X = 2.5
sin X = 2.5 / 5 = 0.50
X = 30 degrees South of East

by the way East speed made good across = 5 cos 30