Suppose that a and b are integers, a = 4 (mod 13), and b = 9 (mod 13). Find the integer c with 0 <=c<=12 such that a) c =

9a (mod 13). b) c = 11b (mod 13). c) c= a+b (mod 13). d) c= a^2 + b^2 (mod 13). e) c = a^3 – b^3 (mod 13).