Characteristic Line of a Sound Wave??

Lucus

Okay, I have another fluids question:

The velocity (u) caused by a rightward propagating wave in a gas is described by the nonlinear wave equation
(du/dt) + [a + ((gamma+1)*u/2)]*(du/dx) = 0,
where a is the speed of sound, gamma is a constant, and (du/dt) and (du/dx) are partial derivatives. I need to derive the equation for the characteristic line eta(t,x) and I don't really know where to start. Any help would be greatly appreciated. Thanks!