(c) For third line of Balmer series $n_{1} = 2,n_{2} = 5$
$\therefore\frac{1}{\lambda} = RZ^{2}\left\lbrack \frac{1}{n_{1}^{2}} - \frac{1}{n_{2}^{2}} \right\rbrack$ gives $Z^{2} = \frac{n_{1}^{2}n_{2}^{2}}{(n_{2}^{2} - n_{1}^{2})\lambda R}$
On putting values $Z = 2$
From $E = - \frac{13.6Z^{2}}{n^{2}} = \frac{- 13.6(2)^{2}}{(1)^{2}} = - 54.4eV$