Recall the proper formula of Young's moduls in terms of stress and strain, 

$Y=\frac{stress}{strain}$, here stress is defined as force per unit cross-section area and strain is the ratio of increment of lenght and original lenght, 

$$\begin{gathered} \Rightarrow Y=\frac{stress}{strain}=\frac{{\displaystyle \frac{mg}{A}}}{{\displaystyle \frac{∆x}{L}}}\Rightarrow ∆x=\frac{{\displaystyle mgL}}{{\displaystyle AY}}=\frac{500\times 10\times 1}{0.50\times {10}^{-4}\times {10}^{11}} \\ \Rightarrow ∆x=1 \mathrm{mm} \end{gathered}$$