Argumentative essays are about presenting your belief on something, supporting a side, presenting the opinions and facts regarding the other side, and then presenting the arguments you have against those opinions and facts. The purpose of an argumentative essay is to persuade something about the topic that you've chosen, so it's best if you answer persuasive.
The inclusion of pro-slavery Missouri representatives would give the pro-slavery party a Congressional majority, in addition to the broader moral question raised by the growth of slavery. The Missouri Compromise was the result of a series of compromises reached by Congress.
