Since this is a three-hour round trip if he spends t hours paddling downstream he must spend 3 - t hours paddling upstream.
Going downstream he travels at 3 + 2 = 5 mph. Traveling at 5 mph for t hours how far does he go?
Going upstream he travels at 3 - 2 = 1 mph. Traveling at 1 mph for 3 - t hours how far does he go?
Since the is a round trip the distance he travels downstream is the same as the distance he travels upstream.