$ = - \frac{{13.6 \times {Z^2}}}{{16}} + \frac{{13.6 \times {Z^2}}}{9}\,\, = \,\, - 13.6 \times 25\left[ {\frac{{ - 1}}{9} + \frac{1}{{16}}} \right]\, = 16.5\,\,eV$