show that if f1(x) and g1(x) are congruent as polynomials modulo n and f2(x) and g2(x) are congruent as polynomials modulo n, then

a) (f1+f2)(x) and (g1+g2)(x) are congruent as polynomials modulo n.

b) (f1f2)(x) and (g1g2)(x) are congruent as polynomials modulo n.

thanks in advance.