Solve the partial differential equation.

Question

The solution of the following partial differential equation $\frac{\partial^2 u}{\partial x^2} = 9\frac{\partial^2 u}{\partial y^2}$ is

1. $\sin (3x-y)$
2. $3x^2 - y^2$
3. $\sin (3x - 3y)$
4. $(3y^2 - x^2)$