$\displaystyle w(x,t)=e^{x-ct}$

Thank you for the help on the last problem, but for this one I'm still stumped as to what I need to do, even after reading a bit on wikipedia. Any help would be much appreciated.

Wikipedia has:

$\displaystyle \frac{\partial u^{2}}{\partial t^{2}}=c^{2}\nabla^{2}u$

But, I don't know what u and $\displaystyle \nabla$ are.