Suppose we have the 3D wave equation
$u_{tt} = 9 (u_{xx} + u_{yy} + u_{zz})$
$-\infty < x,y,z < \infty$ and $t > 0$.
Suppose that
$u(x,y,z,0) = 6 \sin(x+y+z)$
$u_t(x,y,z,0) = 0$

Find $u(x,y,z,t) = {}$
Hint: It is a planar wave. Make sure to test your proposed solution, the constants may be somewhat different that you first think.

help (formulas)