Answer
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.
