Answer
$P = 0$
Work Step by Step
When the phase difference is $~~\pi~rad,~~$ there is fully destructive interference. In this case, the amplitude is zero.
We can find the average power:
$P = I~A$
$P = \frac{1}{2}\rho~v~\omega^2~s_m^2~\pi~r^2$
$P = (\frac{1}{2})(1.21~kg/m^3)(343~m/s)~(3000~rad/s)^2~(0)(\pi)~(2.00\times 10^{-2}~m)^2$
$P = 0$
