7.9

E_{n} = - \frac{2, 17 \cdot 10^{- 18}}{n^{2}} J = - \frac{13, 6}{n^{2}} e V

${E_n= - \frac {2,17 \cdot 10^{-18}} {n^2} J=- \frac {13,6} {n^2} eV}$