Two loudspeakers are placed on a wall 2.00 m apart. A listener stands 3.00 m from the

Question:

Two loudspeakers are placed on a wall 2.00 m apart. A listener stands 3.00 m from the wall directly in front of one of the speakers. A single oscillator is driving the speakers at a frequency of 300 Hz.
(a) What is the phase difference between the two waves when they reach the observer?
(b) What If? What is the frequency closest to 300 Hz to which the oscillator may be adjusted such that the observer hears minimal sound?