Answer :
We know that the momentum is conserved this means that:
[tex]p_{1i}+p_{2i}=p_{1f}+p_{2f}[/tex]where
[tex]p=mv[/tex]In this case the balls are moving opposite each other this means that the velocities have opposite sign, we also know that the 3 kg ball stops after the collision this means that it final velocity is zero, then we have:
[tex]\begin{gathered} (3)(2)-(1)(2)=(3)(0)+1v_{2f} \\ 6-2=v_{2f} \\ v_{2f}=4 \end{gathered}[/tex]Therefore the velocity of the 1 kg ball after the collision is 4 m/s