$12 \mathrm{eV}$ energy is given to electron in third orbit of $\mathrm{H}$-atom, then final energy of electron when it ionise from third orbit, is

Given, total energy given to electron in third orbit,

$E=12 \mathrm{eV}$

Energy of third orbit of $\mathrm{H}$-atom, $E\text{'}=\frac{-13.6}{n^2}$

$=\frac{-13.6}{3^2}=-1.51 \mathrm{eV}$

lonisation energy of electron of third orbit, $E^{\text{'}\text{'}}=-E\text{'}$

$=-\left(-1.51 \mathrm{eV}\right)\Rightarrow E^{\text{'}\text{'}}=1.51 \mathrm{eV}$

Hence, final energy of electron when it comes out of $\mathrm{H}$-atom from third orbit

$E_f=E-E^{\text{'}\text{'}}$

$=12-1.51=10.49 \mathrm{eV}$