A:Answer If you still have this at least 3-year-old TV and it's still being used, two things, you can have a board put in between studs in the wall. They should be 16" or 24" on-center. I think it's doubtful that there aren't any studs on/behind the wall of your 75" TV. However, get a stud finder and locate whatever studs there may be and mount a 2"x 2", 2" x 3", or a 2" x 4" board between whatever studs you can locate, then attach a TV wall hanger (of your choice and design) on the studs and subsequently hang your TV with the equipment that is supplied with the wall hanger.
Secondly, if you do have a working fireplace, I wouldn't recommend placing your TV over it under any circumstance due to the heat and possible smoke given off/from the fireplace when used.
Thirdly, if you're writing to and from a Best Buy input, either hire them or another company to hang your TV. If you buy a large TV of this nature, Best Buy will deliver and hang it for free as they have recently done for me.