The force F1 exerted by girl is the ‘action’ and the force F2 exerted by the boat is the ‘reaction’.
Now, momentum of the girl in one direction must be equal to the momentum of the boat in opposite direction.
m1 × v1 = m2 × v2
⇒ 50 × 3 = 300 × v2
⇒ v2 = 0.5
Hence, velocity with which the boat begins to move backwards is 0.5 m/s.