Suppose that a and b are integers, a ≡ 11 (mod 19), and b ≡ 3 (mod 19). Find the integer c with 0 ≤ c ≤ 18 such that a) c ≡ 13a (mod 19). b) c ≡ 8b (mod 19). c) c ≡ a − b (mod 19). d) c ≡ 7a + 3b (mod 19). e) c ≡ 2a2 + 3b2 (mod 19). f ) c ≡ a3 + 4b3 (mod 19).