$$U_{tt} = \Delta U + x^3 - 3xy^2$$
$$U|_{t=0} = e^x \cos y$$
$$U_t|_{t=0} = e^y \sin x$$
Help me, please, with solution of this equation. Can you prompt me algorithm to find the solution? I know, that I have to use Poisson's formula, but I can't integrate it.