Agh, I've been working on this question for an hour and can't seem to make any headway. I seem to be completely lost on it.

a.) Show that the positive integers less than 11, except 1 and 10 can be split into pairs of integers such that each pair consists of integers that are inverses of each other modulo 11.

b.) use part (a) to show that 10! = -1(mod 11)

Any help would be appreciated