I dont know what this question is asking me.

It uses a symbol, an upside down U, apparently meaning intersection of sets. I have yet to deal with such an integers modulo n problem:

Let a and b be integers.

Prove that either [a]n(upside down U)[b]n = 0 or [a]n=[b]n

Note that the n's are subscripted. (congruence classes of modulo n)