Let $\Omega =\mathbb{R}^d \setminus \overline{B(0,1)}$. Suppose $u \in C^2(\Omega;\mathbb{R})$ is a harmonic function. Consider, for all $x \in \mathbb{R}^d$, the function, $\phi: (|x| + 1, \infty) \to\mathbb{R}$ given by,
$$\phi(r) = \frac{1}{|\partial B(x,r)|} \int_{\partial B(x,r)} u(y) \mathrm{d}S(y) =\def\avint{\mathop{\,\rlap{-}\!\!\int}\nolimits} \avint_{\partial B(x,r)} u(y)\,dS_y $$
Prove that $\phi(r) = A\int_{|x|+1}^{r} s^{1-d}dS + B$. Where A and B are constant depending on $x$ not on $r$.