Answer
$$338 \mathrm{\ MeV} $$
Work Step by Step
$\text { Conservation of energy (see Eq. } 37-47)$ leads to
$K_{f}=-\Delta m c^{2}+K_{i}=\left(m_{\Sigma^{-}}-m_{\pi^{-}}-m_{n}\right) c^{2}+K_{i} $
$\quad=1197.3 \mathrm{MeV}-139.6 \mathrm{MeV}-939.6 \mathrm{MeV}+220 \mathrm{MeV} $
$\quad=338 \mathrm{\ MeV} $
