In Yau and Schoen's differential geometry,in Ch5 before Thm 3.5,the author says

When $R$(scalar curvature of a manifold M)$>0$,there exists a unique Green's function $G$ to the operator $L=-\Delta+aR$ and $LG_{P}=\delta_{P}$.Here $P$ is an arbitrary point of $M$.

In the normal coordinate of $P$,we have $$ G_P(x)=\frac{1}{(n-2) \omega_{n-1}} r^{2-n}(1+o(1)). $$

I wonder where can I find this existence result and how to get the expansion at point $P$.