Two antennas located at points A and B are broadcasting radio waves of frequency $98\mathrm{.}0$ MHz$,$ perfectly in phase with each other. The two antennas are separated by a distance d$=12\mathrm{.}40$ m$.$ An observer, P$,$ is located on the x axis, a distance x$=56\mathrm{.}0$ m from antenna A$,$ so that APB forms a right triangle with PB as hypotenuse. What is the phase difference between the waves arriving at P from antennas A and B$?$ Now observer P walks along the x axis toward antenna A$.$ What is P$\text{'}$s distance from A when he first observes fully destructive interference between the two waves?