Let the speed of ball be $\mathrm{v}$ and speed of the wedge be ${\mathrm{v}}_{2\mathrm{m}}$

using conservation of momentum 

$0=\mathrm{mv}+2{\mathrm{mv}}_{2\mathrm{m}}$

${\mathrm{v}}_{2\mathrm{m}}=-\frac{\mathrm{v}}{2}$

Now, using work-energy theorem on the system,

$\mathrm{mgR}=\frac{1}{2}{\mathrm{mv}}^2+\frac{1}{2}\left(2\mathrm{m}\right)\times {\left(\frac{\mathrm{v}}{2}\right)}^2$

$\Rightarrow \mathrm{ }\mathrm{v}=\sqrt{\frac{4\mathrm{gR}}{3}}$