Hello. I don't know what evidence you mentioned in your previous assignment, which makes it impossible for me to answer your question efficiently. However, I will try to help you in the best possible way, showing you how to justify the chosen doubts.
you must justify the evidence based on the importance it has in the story you read. If you selected evidence where Douglass reflects on the importance of education, you can justify it by using the fact that education is liberating and would allow slaves to have enough knowledge to fight for themselves. If you used evidence about the misrepresentation that slavery imposes on society, you can justify how slavery is based on dominance and humiliation, generating results as depressing as the process itself.