Answer
$\frac{h}{\sqrt{2mqV}}$
Work Step by Step
We know that
$\frac{1}{2}mv^2=qV$
$\implies \frac{m^2v^2}{2m}=qV$
$\implies \frac{(mv)^2}{2m}=qV$
$\implies \frac{p^2}{2m}=qV$
But $p=\frac{h}{\lambda}$
$\implies \frac{(\frac{h}{\lambda})^2}{2m}=qV$
This simplifies to:
$\lambda=\frac{h}{\sqrt{2mqV}}$
