A square is 3.5 m on a side, and point A is the midpoint of one of

Question:

A square is 3.5 m on a side, and point A is the midpoint of one of the sides. On the side opposite this spot, two in-phase loudspeakers are located at adjacent corners, as shown in the figure. Standing at point A, you hear a loud sound because constructive interference occurs between the identical sound waves coming from the speakers. As you walk along the side of the square toward either empty corner, the loudness diminishes gradually but does not entirely disappear until you reach either empty corner, where you hear no sound at all. Thus, at each empty corner destructive interference occurs.
Concepts:
(i) Why does constructive interference occur at point A?
(ii) What is the general condition that leads to destructive interference?
(iii) The condition that leads to destructive interference entails a number of possibilities. Which of them applies at either empty corner of the square?
Calculation: Find the wavelength of the sound waves.