16.34

The latter parts of this question will make more sense after tomorrow's lecture, if they don't from reading the chapter. To get you started:

The average power transmitted along the string is

P = \frac{1}{2}\mu v \omega^2 y_m^2

Here you don't know the velocity, but you can relate it to the tension and linear density.

If you superimpose two equal-amplitude sine waves together of the same frequency (so kx-wt is the same for both), but they differ only by a phase phi, the resulting wave is given by Eq. 16-51:

y(x,t) = \left[2y_m \cos{\left(\frac{\varphi}{2}\right)}\right]\sin{\left(kx-\omega t + \frac{\varphi}{2}\right)}

The amplitude then depends on the relative phase as well as the amplitude of each wave. If the waves are in-phase, the total amplitude is just double, but if they are 180 degrees out of phase, everything cancels.