Please help

While testing speakers for a concert, Tomas sets up two speakers to produce sound waves at the same frequency, which is between 100 Hz and 150 Hz. 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 26.6 m from one speaker and 37.8 m from the other. (The speed of sound in air is 343 m/s.) What is the maximum frequency of the sound waves coming from the speakers? Hz