There can be two definitions of “competitive multi-party system.” Some scholars use the term “multi-party” to refer to any system in which there is more than one political party. Others use it to mean any system where there are more than two political parties. That is, by some definitions the United States would be a multi-party system while by others it would not be because it only has two political parties. The “competitive” part remains the same in both definitions. In a competitive system, the parties have a chance to compete for actual power in elections. In other words, the elections are not just for show. Opposition parties have a real chance of gaining power.
As for “our 6/6 positive and negative points” about these systems, you will need to consult your book or your class notes. We cannot know what six positive and negative points have been mentioned in your class. Below, you can find a link that discusses pros and cons of two-party systems versus multi-party systems. In general, the main positive thing about a multi-party system is that it allows for a broad variety of viewpoints to be represented. People can choose between many parties to find the one that has the policies that best fit their beliefs. By contrast, in a two-party system, it is unlikely that either party’s policies will conform exactly to one’s own beliefs. The main drawback of a multi-party system is that it is very fragmented and does not encourage compromise as much as a two-party system does. When two parties have to compete to win a majority of the votes, they have to create policy platforms that appeal to a broad variety of people. This encourages more inclusive policy platforms than might be seen in parties in a multi-party system.