# General solution of wave equation [duplicate]

A vector potential $A(x, t)$ satisfies the wave equation $\left(\Delta-\frac{1}{c^2}\frac{\partial^2}{\partial t^2}\right) A(x, t)=0$.
It is written in my physics textbook that the general solution of the wave function is superposition of $e^{(1)}a_k e^{i(k\cdot x-\omega t)}$.