How to convert this into eV

Find out the energy levels in Joules and simply use the conversion between Joules and eV. Should be easily google'd/wikipedia'ed or should be in your textbook.

 

There are some useful combinations of the fundamental constants. If you know them, you can make calculations a lot faster.
\begin{align*}
h c &= 1240~\text{eV nm} \\
\hbar c &= 197~\text{eV nm} = 197~\text{MeV fm} \\
m_e c^2 &= 511000~\text{eV} = 511~\text{keV} = 0.511~\text{MeV}
\end{align*}
For your problem, you just need to introduce factors of c, the speed of light, to form the right combinations:
[tex]\frac{h^2}{8m} = \frac{(hc)^2}{8mc^2} = 0.376~\text{eV nm}^2[/tex]Note that this combination has two powers of length. They are needed to cancel the units from a² in the denominator of the formula for the energy.