If the first excitation energy of a hydrogen-like atom is 27 . 3eV...

First excitation energy

$= {Z}^{2}Rhc\left(\dfrac{1}{{1}^{2}} - \dfrac{1}{{2}^{2}}\right) = {Z}^{2}Rhc\dfrac{3}{4}$

$∴ \dfrac{3}{4}Rhc = 27.3 eV$

$∴ {Z}^{2}Rhc = \dfrac{4}{3} 27.3 eV = 36.4 eV$