evans book pde estimate question

Hint: use polar coordinates to integrate the green's function on the $\epsilon$ ball.

Edit: Observe for $n\geq 3$ we have \begin{align} \int_{B(0, \epsilon)} \frac{dx}{|x|^{n-2}} = \int^\epsilon_0 \int_{|x|=r} \frac{dS(x)}{|x|^{n-2}}\ dr = C\int^\epsilon_0 \frac{r^{n-1}}{r^{n-2}}\ dr = C'\epsilon^2. \end{align}