Use the conservation of momentum principle. The momentum before the collision equals the momentum after the collision. $$(40 kg)(0) + (15 kg)(3.0 m/s) = (40.0 kg + 15 kg) v$$ $$v = 0.8 m/s$$
Someone from the community is currently working feverishly to complete this textbook answer. Don’t worry, it shouldn’t be long.