Answer
$7.01\times10^{-15}m$.
Work Step by Step
$\lambda=\frac{h}{p}=\frac{h}{\sqrt{2mE}}$
$\lambda=\frac{6.626\times10^{-34}J \cdot s}{\sqrt{2(6.64\times10^{-27}kg)(4.20\times 10^{6}eV)(1.60\times10^{-19}J/eV)}}$
$\lambda= 7.01\times10^{-15}m$
