the transformation of Laplace equation

Question

If the 2D Laplace equation is invariant under the following transformations: $x'^{i}=\frac{x^{i}-b^i x^2}{1-2\vec{b} \cdot \vec{x}+b^{2}x^2 } $ and u'(x')=u(x) . $b^{i}$ is a two-dimensional constant vector.I don't know how to prove this conclusion.