along \(x\)-axis no internal force exists, hence momentum will be conserved along \(x\)-axis

\(\left.\left.P_i\right)_x=P_f\right)_x\)

\((m+m+2 m) \cos \theta=-m u \cos \theta+0+2 m V\)

\(\Rightarrow 2 m v=5 m u \cos \theta\)

\(v=\frac{5}{2} u \cos \theta\)

and along \(y\) direction \(\left.P_i\right)_y=0\) So \(\left.P_f\right)_y=0\)