Bought a car under false pretenses

I was ecplained that the car has an electrical problem and that the battery was not staying charged. Had electrical problem fixed amd bought a new battery. That was fine. I was however never told about the transmission going bad and it started acting up on my way home, after it got warm. My mechanic has told me its my transmission and that the problem was pre existing, meaning the lot knew about it and never told me. What can i do