Problem 1274 (difficulty: 2/10)

Let \(\displaystyle f(x,y)=\psi(x-ay)+\phi(x+ay)\), where \(\displaystyle \psi,\phi\) are smooth.

\(\displaystyle {\partial^2 f \over \partial y^2} - a^2 {\partial^2 f\over \partial x^2}=?\)

Give me another random problem!