Recall the condition of conservation of linear momentum in $x-$axis is,

$$\begin{gathered} 0=M\times 4+0+2M\times u\cos \theta \\ u\cos \theta =2...\left(1\right) \end{gathered}$$

and a vertical component,

$$\begin{gathered} 0=0+6M+2Mu\sin \theta \\ u\sin \theta =3...\left(2\right) \end{gathered}$$

from equation $\left(1\right) \& \left(2\right)$

$$\begin{gathered} \Rightarrow u^2{\cos }^2\theta =4 \& u^2{\sin }^2\theta =9 \\ \Rightarrow u^2{\cos }^2\theta +u^2{\sin }^2\theta =13 \\ \Rightarrow u^2=13 \\ \Rightarrow u=\sqrt{13} \mathrm{m} {\mathrm{s}}^{-1} \end{gathered}$$