(b) Energy $E = K\left\lbrack \frac{1}{n_{1}^{2}} - \frac{1}{n_{2}^{2}} \right\rbrack$ ($K$ = constant)
$n_{1} = 2$ and $n_{2} = 3$, so $E = \left\lbrack \frac{1}{2^{2}} - \frac{1}{3^{2}} \right\rbrack = K\left\lbrack \frac{5}{36} \right\rbrack$
For removing an electron, $n_{1} = 1$ to $n_{2} = \infty$
Energy $E_{1} = K\lbrack 1\rbrack = \frac{36}{5}E = 7.2E$
$\therefore$ Ionization energy $= 7.2\ E$