The wavelength is given by
\[\lambda = \frac{h}{p}\]
where
\[h=6.626 \times 10^{-34} Js\]
is Planck's constant and \[p\]
is the electrons momentum.
The wave equation is \[v = f \lambda\]
and the mometum is \[p=mv\]
so that \[\frac{v}{f} = \frac{h}{mv} \rightarrow mv^2 = hf \]
But
\[KE=\frac{1}{2}mv^2\]
so
\[KE=\frac{1}{2} hf\]
for an electron.
For an electron wtih a freuency
\[10^20 Hz\]
\[KE=\frac{1}{2}hf=\frac{1}{2} \times 6.626 \times 10^{-34} \times 10^20 =3.313 \times 10^{-14} J\]
.