Answer
$E=3.94\times 10^{-18}J$
Work Step by Step
We can determine the ionization potential energy of helium as follows:
$E=\frac{hc}{\lambda}$
We plug in the known values to obtain:
$E=\frac{6.625\times 10^{-34}J.s(3\times 10^8m/s)}{(50.4\times 10^{-9}m)}$
We plug in the known values to obtain:
$E=3.94\times 10^{-18}J$