We know that $ y=-6 $ from the given information. By the distance formula, we have the equation $ \sqrt{(x-8)^2+(-6-3)^2}=15 $. Solving, we have \begin{align*} \sqrt{(x-8)^2+(-6-3)^2}&=15 \\ x^2-16x+64+81&=225 \\ x^2-16x-80&=0 \\ (x-20)(x+4)&=0 \end{align*}Thus, $ x+4=0 $ or $ x-20=0 $, so $ x=-4 $ or $ x=20 $. $ x=-4 $ by the given conditions. Thus, our point is $ (-4,-6) $ and is a distance of $ \sqrt{(-4)^2+(-6)^2}=\sqrt{52} $ units from the origin. $ n=\boxed{52} $.