The second thing I'd do is check your book's index and look through class notes.
The third thing I'd do, if you don't have a friend in the class, is ask the professor by email: "Can you define positive set? I missed that class, and can't find the definition in the book or online."
This is really something you should jump on quickly, as soon as you realize you don't have what you need to complete an assignment.
(Also, I finally realize why I didn't gather what "+" was supposed to stand for...the other poster didn't put up the cardinality notation, so "+" was addition on natural numbers, not a set operation. Derp.)