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