One of the standard "rules" of relational database design is that every table should have a primary key. If you have a M:N relationship between tblX and tblY, then you'll need to create a bridge table. There are a couple of ways to handle the primary key for the bridge. You can either use the tblX and tblY primary keys as a composite primary key of the bridge or you can make up an entirely new field for the bridge primary key.
There are advantages and disadvantages to both approaches. Sometimes, we should use a composite PK instead of a simple PK. Sometimes, we can't use a composite PK.
Either style of PK will be fine for this assignment.