2D linear inhomogeneous wave equation with inhomogeneous time-independent initial conditions I'm looking for any insight into solving the following PDE: 
$$u_{tt}=c^2 (u_{xx}+u_{yy})-\sin(y)$$
$$u=0, y\in {0,\pi} $$
$$u_x=0, x\in {0,1}$$ 
$$u(x,y,0)=\cos(\pi x)\sin(3y) $$
$$u_t(x,y,0)=0$$
In particular, if I was to attempt a Fourier series solution to the problem, given that both the equation and initial conditions are inhomogeneous, what is the thought process of guessing a solution form? I tried guessing a form like 
$$ \sum\limits_{i=1}^\infty A_n(y,t)\cos(\pi nx) $$
then I tried to find the coefficients $A_n$ by matching the initial conditions first, and then the boundary conditions, but I couldn't get this to lead anywhere. Any help would be much appreciated! 

The solution is
$u \left( x,y,t \right) =\cos \left( \pi \,x \right) \sin \left( 3\,y
 \right) \cos \left( c\sqrt {{\pi }^{2}+9}t \right) +{\frac {\sin
 \left( y \right) \cos \left( ct \right) }{{c}^{2}}}-{\frac {\sin
 \left( y \right) }{{c}^{2}}}
$
