Momentum — practice question

Two small smooth spheres $A$ and $B$, with equal radii and masses $4\,\text{kg}$ and $m\,\text{kg}$ respectively, are on a smooth horizontal plane. At the start, sphere $B$ is stationary and $A$ is moving towards $B$ at $6\,\text{m s}^{-1}$. Following the collision $A$ has speed $1.5\,\text{m s}^{-1}$ and $B$ has speed $3\,\text{m s}^{-1}$.