Answer: $\large\frac{M+m}{m}$$ \sqrt {2gh} $

Per law of conservation of linear momentum $mV=(m+M)v_{\large system}$-----(1)

Per law of conservation of energy $\large\frac{1}{2} $$(m+M)v_{\large system}^2=(m+M)gh$

$\Rightarrow v_{\large system}=\sqrt {2gh}$

From (1) we get Therefore $V=\bigg(\large\frac{m+M}{m}\bigg) $$\sqrt {2gh}$