I'm going to cheat and give you two takes on the question.
1. the economic effect of not having to pay servants by having slaves instead allowed the US to become a strong and independent country when it might not otherwise have been possible, or sooner than it might have happened
2. I think the effect on American society of the history of having taking advantage of slave labour continues to be enormous. The friction between races in America is more pronounced than it is in some other developed countries and I believe it comes from that background.
I hope those are short enough
Good luck with your assignment!