Interference and frequencies...?

I'll not do YOUR work for you. Based on the premise given, if the listener is in the right place then the waves produced by the 2 speakers will cancel each other and produce no sound, or a very attenuated sound. For the frequency range given, and only single tones used, cancellation can occur at any frequency. For a complex wave, such as that produced by music, then and frequency that is emitted by the speakers at the same time, will cancel each other out in free space. It does work, now go do your home work.