Calculate the momentum before the collision. $$(2 kg)(3 m/s) + (2 kg)(0 m/s) = (2 kg)(3 m/s)$$ Calculate the momentum afterward. $$(2 kg)(v) + (2 kg)(v) = (4 kg)(v)$$ Momentum is consumed in this collision, meaning it does not change. Set the two expressions equal and solve for v = 1.5 m/s. This is discussed on page 99.