Suppose that a and b are integers, a ≡ 11 ( mod 19), and b ≡ 3 ( mod 19 ) .find integer c with0 ≤ c ≤ 18 such that

a.c ≡ 13 a ( mod19).

b.c ≡ 8 b( mod19).

c.c ≡ a − b( mod19).

d.c ≡ 7 a + 3 b( mod19).

e.c ≡ 2 a 2 + 3 b 2 ( mod19). f) c ≡ a 3 + 4 b 3 ( mod19).