We can't answer this without knowing how tall the bridge is, if we knew that, we could use kinematics to get the time it takes for the stone to fall, then take the distance covered by the boat between when the stone was dropped and when it hit the water (27m - 3m = 24m) and divide that distance by the time it took for the stone to fall to the water and we'd have the average speed of the boat in that time (v = d/t works here because acceleration is 0, so no more complicated kinematics is necessary). I don't see how this can be answered without some notion of time or the height of the bridge being given.
