Let $u$ be a solution of the wave equation $-u_{tt}+\nabla^2u =0$ with the initial conditions $u(0,x)=0$ and $u_t(0,x)=h(x) = h_1(\vert x \vert)$
Show that $$u(x,t)=\int_{\Vert x \Vert -t}^{\Vert x \Vert +t}{r h_1(r)}dr$$
Kirchhoff's formula would imply that $$ u(x,t)=\frac{1}{4\pi t^2}\int_{\Vert x-y \Vert =t}{t h_1(\Vert y \Vert)dy} $$
How can I go from here ? I tried finding a good substitution for $y$ but that didn't help me.
Woud appreciate any help/hints