Distance = rate * time (d=rt)
Since the boat travels up and down stream the total distance is 48km. As far as the river current, because the boat goes up and down that 3km/hr will cancel itself out. The boat will be slowed down in one direction but spead up in the other direction
So, just use d=48, t=6 and solve for r
48=6r
r=8
8km/hr