The law of conservation of momentum is given as

$m_{1}u+m_{2}u\text{'}=\left(m_1+m_2\right)v\text{'}$

The given data is 

$$\begin{gathered} u=v \\ u\text{'}=0 \mathrm{m} {\mathrm{s}}^{-1} \\ {m}_1\mathit{=}m \\ {m}_2\mathit{=}2m \end{gathered}$$

Using the conservation of momentum equation,

$$\begin{gathered} mv+2m\left(0\right)=(m+2m)v\text{'} \\ \Rightarrow v\text{'}=\frac{mv}{3m}=\frac{v}{3} \end{gathered}$$