Part A) The wavelength of a sound wave can be determined by the distance between two consecutive points of the wave that are in phase, or have the same displacement and direction. In this case, the distance between the two loudspeakers is given as 15 cm when the sound has maximum intensity.
Since the sound waves are emitted along the x-axis, we can assume that the distance between the two speakers represents one complete wavelength. Therefore, the wavelength of the sound is 15 cm.
Part B) If the distance between the speakers continues to increase, the sound intensity will again be a maximum when the separation between the speakers is equal to an integer multiple of the wavelength.
Since the wavelength is 15 cm, the sound intensity will again be a maximum when the separation between the speakers is a multiple of 15 cm. For example, when the separation is 30 cm (2 wavelengths), 45 cm (3 wavelengths), 60 cm (4 wavelengths), and so on.