de Broglie wavelength associated with the particle $\lambda=\frac{ h }{ p }$ where $h$ is the Planck's constant now we know thwt, $E =\frac{1}{2} mv ^{2}$ and $p = mv$ by mutiplying by $m$ on both sides we get,
$p =\sqrt{2 mE }$
hence,
$
\lambda=\frac{ h }{\sqrt{2 mE }}
$