I want to solve this wave equation in $3$-D using Kirchoff's formula but all I've done it doesn't work. In particular I'm not able to compute the integral in Kirchoff's formula. you can see the cauchy pb in the image. enter image description here
if you cannot see the image the Cauchy problem, it is this the classical one (homogeneous problem) with $c=1$ and $F=0$ (no external force) and $u(x,0)=|x|^2$ and $u\, t(x,0)=0$.