Answer and Explanation:
Hello. You did not provide the posters and texts that were read in class. This will make the examples and evidence provided here somewhat inaccurate.
1. A primary source is a document or media that represents an account of a particular historical period and was written while that period was taking place. In this case, we can say that the posters about slavery that you saw in the classroom, were created at the time that slavery is happening, being then, a primary source.
2. Slavery was an American economic engine, as it was the main driving force of the agricultural sector, mainly in cotton production, which the United States was the largest producer and exporter, which yielded many incomes for the American economy.
3. There are many examples that reinforce that slavery did happen in the country. Among them we can mention, the narrative of the life of Federick douglass, the journey of a fugitive and several other reports of ex-slaves.