Need help with Combinatorics Problem!!!!

(a) Let i, j be positive integers. Prove that if i # j, then there exist two binary strings s, t where s has length i and t has length j such that { s, t)* is an ambiguous expression of binary strings. (b) Prove that if s and t are distinct binary strings where the length of s is the same as the length of t, then { s, t}* is an unambiguous expression of binary strings