Please give me some hints to solve the equation

$\displaystyle \frac{\partial^2 z}{\partial x^2}+\frac{\partial^2 z}{\partial y^2}=k^2 z$

Boundary condition: $\displaystyle z=z_0(\mbox{constant}) \quad\mbox{at}\quad x=\pm a \quad\mbox{and}\quad y=\pm b$