Explain what did the United States government do to sell off the last part of the land for settlement

I’m not sure what time period you are talking about, but the “last part” if the land for settlement you must be talking about the Native American land, the US government sold the land to settlers behind the the Native Americans back, and they sold even though they had a treaty and the Americans “gave” the land to the native Americans for reservations.

The founders also specified a process by which the Constitution may be amended, and since its ratification, the Constitution has been amended 27 times.