Momentum — practice question

Two small smooth spheres $A$ and $B$, with equal radii and masses $km\ \text{kg}$ and $m\ \text{kg}$ respectively, where $k > 1$, are able to move freely on a smooth horizontal plane. $A$ moves towards $B$ at speed $6\ \text{m s}^{-1}$, and $B$ moves towards $A$ at speed $2\ \text{m s}^{-1}$. Following the collision, $A$ and $B$ coalesce and then travel with speed $4\ \text{m s}^{-1}$.