Distance = speed x time
Distance (From departure point TO..Guam) = speed x 3 h
Distance (FROM Guam to departure point) = 6 km/h x 4h = 24 km
But Distance (From TO) = Distance (TO From) = same, then
Speed x 3 = 6 x 4 → Speed x 3 = 24 and Speed = 24/3 = 8 km/h