From Mechanics

Using

\displaystyle {{m}_{A}}{{v}_{A}}+{{m}_{B}}{{v}_{B}}={{m}_{A}}{{u}_{A}}+{{m}_{B}}{{u}_{B}}

\displaystyle \begin{align} & 3\times 4+5\times (-2)=3\times 0+5v \\ & 12-10=5v \\ & v=\frac{2}{5}=0\textrm{.}4\text{ m}{{\text{s}}^{\text{-1}}} \\ \end{align}

Continues to move in same direction at a speed 0.4 \displaystyle \text{m}{{\text{s}}^{\text{-1}}}.