$\lambda=\frac{\mathrm{hv}}{\mathrm{mv}^2}$

$\frac{\mathrm{mv}^2}{\mathrm{~h}}=\frac{\mathrm{v}}{\lambda}=\mathrm{v} \text { (frequency) }$

$\text { Given } \frac{1}{2} \mathrm{mv}^2=2.18 \times 10^{-18} \mathrm{~J}$

$\mathrm{~h}=6.6 \times 10^{-34}$

$\mathrm{v}=\frac{4.36 \times 10^{-18}}{6.6 \times 10^{-34}}=660.60 \times 10^{13} \mathrm{~Hz}$

$\approx 661 \times 10^{13} \mathrm{~Hz}$