Answer
$v=.268 \ Mm/s$
Work Step by Step
We first use the equation for kinetic energy to find the speed of the alpha particle. Thus:
$v = \sqrt{\frac{2K}{m}}= \sqrt{\frac{2(5.15\times10^6)}{4 \ amu}}$
We know that momentum is conserved, so it follows:
$239v+4(\sqrt{\frac{2(5.15\times10^6)}{4 \ amu}})=0$
$v=.268 \ Mm/s$
