(II) An atomic nucleus initially moving at 420 $\mathrm{m} / \mathrm{s}$ emits an alpha particle in the direction of its velocity, and the remaining nucleus slows to 350 $\mathrm{m} / \mathrm{s}$ . If the alpha particle has a mass of 4.0 $\mathrm{u}$ and the original nucleus has a mass of $222 \mathrm{u},$ what speed does the alpha particle have when it is emitted?