$${K_{\mu} = \frac{(33,9\, \mathrm{MeV})^2}{(2)(33,9\, \mathrm{MeV} + m_{\mu}c^2)} \\ = \frac{(33,9\, \mathrm{MeV})^2}{(2)(33,9\, \mathrm{MeV} + 105,7\, \mathrm{MeV})} \\ = \underline{\underline{4,12\, \mathrm{MeV}}}}$$