The element ${ }_{84}^{210}\mathrm{Po}$ emits an alpha particle. The daughter element is 

The formula for the reaction is as follows:  ${ }_{84}{\mathrm{Po}}^{210}{\to }_{82}{\mathrm{Pb}}^{206}{+}_2{\mathrm{He}}^4$ 

Particle mass is 4m, where m is the mass of a nucleon. Because the parent nuclei were at rest, the offspring nuclei's mass will be 206 m, and their net momentum should be zero.
therefore if daughter nuclei move at a speed of V,

$\mathrm{V}\left(206\mathrm{m}\right)+4\mathrm{m}\left(\mathrm{v}\right)=0$ V's magnitude will then be $\mathrm{V}=103 2\mathrm{V}$.