Question

What is the energy in eV of the electron of a hydrogen atom for the orbit designated n = 3?

Answer 1

you definitely know about Bohr's atomic theory.
what is formula of energy of the electron of a hydrogen atom for the nth orbit ?
of course, $\bf{E_n=-\frac{13.6}{n^2}eV}$
Here n denotes nth orbit and En donetes energy of electron in nth orbit of hydrogen atom.

for n = 3 ,
$\bf{E_3=-\frac{13.6}{3^2}}\\=-\frac{13.6}{9}=-1.51eV$