Problem $11\mathrm{.}058-$ Frequencies of the sound waves coming from two speakers

While testing speakers for a concert, Tom$$s sets up two speakers to produce sound waves at the same frequency, which is between $100Hz$ and $150Hz.$ The two speakers vibrate in phase with each other. He notices that when he listens at certain locations, the sound is very soft $($a minimum intensity compared to nearby points$).$ One such point is $27\mathrm{.}1m$ from one speaker and $38\mathrm{.}1m$ from the other. $($The speed of sound in air is $343\frac{m}{s}.)$

Problem $11\mathrm{.}058-2-$ Maximum frequency of the sound waves

What is the maximum frequency of the sound waves coming from the speakers?

$Hz$