Determine the truth value of the following statements, then find the contrapositive (You do not need to determine the truth value of the contrapositive). If you believe the statement is true, you need to prove it, otherwise you should provide a counterexample.
(a) If x² ≡ 0 mod 9, then x ≡ 0 mod 9.
(b) If mn is even, then either m is even or n is even.