1. The problem statement, all variables and given/known data 2. Relevant equations 3. The attempt at a solution I've done all the parts easily. I just don't know how to derive the beat frequency fbeat. I cannot provide any workings because I don't know where to begin. I am not asking for an answer. I am asking for a push in the right direction. Give me something to get my brain working on this. I hope the mods will understand my intentions.