To demonstrate interference of sound waves, a buzzer is placed in the center of an empty mailing tube that is 0.80 m long. The sound waves emerge from the two ends of the tube in phase and interfere with each other in the region surrounding the tube. When you stand 4.0 m directly in front of the tube, equidistant from its two ends, you hear a loud sound. If you then move parallel to the tube a distance of 1.3 m, the sound nearly disappears,

(a) What's the wavelength of the buzzer's sound waves?

(b) What's the buzzer's frequency? (Assume the speed of sound in air is 340 m/s) Teacher gave answer a) .49m and b) 694Hz what are the steps?