Radial harmonic functions in $\mathbb{R}^d$

Consider a spherically symmetric potential satisfying the Laplace equation in d dimensions \(( d \geq 3 )\) and vanishing at infinity. Show that this potential can be written as \(\textstyle u ( r ) = { \frac { a } { r ^ { b } } }\) , where a is a constant, and find the value of b.