Find counterexamples to each of these statements about congruences. a) If ac bc (mod m), where

Question:

Find counterexamples to each of these statements about congruences.
a) If ac ≡ bc (mod m), where a, b, c, and m are integers with m ≥ 2, then a ≡ b (mod m).
b) If a ≡ b (mod m) and c ≡ d (mod m), where a, b, c, d, and m are integers with c and d positive and m ≥ 2, then ac ≡ bd (mod m)?