Is there a method to solve this in a simple way(For a multiple choice question).

Question is, If $u$ is a function defined on $R^2$ and satisfies the equation $\partial^2U/\partial x^2$-$\partial^2U/\partial y^2$ then the general solution of the equation is:  

a)u(x,y)= $ 2f(x+2y)+g(x-2y) $  

b)u(x,y)= $(x+y)f(x-y)+g(x-y) $  

c)u(x,y)= $(x+2y)f(x+y)+g(x-y)$  

I know how to solve these normally, but it takes time for a **mcq**.Is there a method I can follow to gain the answer quickly