C. (a) Write an equation for a sinusoidal plane wave in the direction k = - , displaying wavelength x+2 and velocity v in the equation (so k, r, and w won't show up in your answer). Hints: How are k, k, and k related? You'll need to figure out kór as one step in your solution, where r at each point is of course (x + y + z). And remember w = kv. (b) Show that this satisfies the three-dimensional differential wave equation.