Three objects A , B and C are kept in a straight line on a frictionless horizontal surface. These have…

Three objects $\mathrm{A}$, $\mathrm{B}$ and $\mathrm{C}$ are kept in a straight line on a frictionless horizontal surface. These have masses $\mathrm{m}$, $2\mathrm{m}$ and $\mathrm{m}$, respectively. The object $\mathrm{A}$ moves towards $\mathrm{B}$ with a speed $9 \mathrm{m} {\mathrm{s}}^{-1}$ and makes an elastic collision with it. Thereafter, $\mathrm{B}$ makes completely inelastic collision with $\mathrm{C}$. All motions occur on the same straight line. Find the final speed (in $\mathrm{m} {\mathrm{s}}^{-1}$) of the object $\mathrm{C}$.