My advice is to return them if the store will accept them in return.
IMO a reputable store wouldn't sell them to you (or would strongly advise you not to get them) if you have a 75 gallon tank.
A few issues that will come up include physical room for swimming, waste production (a large metabolically active fish producing a lot of waste) and the potential for aggression in a small tank, even though these are said to be among the less aggressive triggers.
I don't know the actual recommended size for this species of trigger so someone who has personal experience with this fish might want to chime in.