Answer
Please see the work below.
Work Step by Step
We know that
$PE_{elastic}=\frac{1}{2}(\frac{EA}{L_{\circ}}dL)$
Thus;
$PE_{elastic}=\frac{1}{2}(\frac{2.0\times 10^6(0.5\times 10^{-4})}{3.0\times 10^{-3}})\times 1.0\times 10^{-3}=1.7\times 10^{-2}J$