How did lawmakers propose to prevent future debates ovrr slavery in new territories?

Depends on the era you are talking about. When slavery was relevant, precautions for the future weren't really taken, it was all about the present and the problems we have now (which means back then). If you are talking about near the end of slavery, the law makers of that time wanted there to be no question about the fact that african americans were people AND americans at that. 

Where are the following ?