Answer
$L=4.5\times10^{27}m$
Work Step by Step
$\lambda=\frac{h}{p}=\frac{h}{mv}=2.7608\times10^{-33}m$
Using the formula $\theta=\frac{1.22\lambda}{D}=\frac{1.22(2.7608\times10^{-33}m)}{3.00\times10^{-3}m}=1.1227\times10^{-30}rad$
$\tan(\theta)=\theta=\frac{x}{2L}$
$L=\frac{x}{2\theta}=4.5\times10^{27}m$
