Out of Phase Speakers

Sarah puts two small speakers $d = 2$ meters apart on one wall of her spacious dorm room. Unfortunately, she fails to note that her speakers are connected out of phase. She plays a stereo rendition of ``the harmonic frequency 340 Hz'' and notices that she can hardly hear it with her chair in the middle a distance $D = 10$ meters away (told you it was a spacious dorm room).

a) What is the smallest distance $y$ she can move her chair up along the wall to be able to hear the sound clearly? (Show explicitly why this is true, don't just write down a formula).

b) What else could she do to enable herself to listen with her chair in the middle (where it is)? Explain algebraically.