Mathematical Cryptology

Let N be a large integer and let K = M = C = Z/NZ. For each of the functions e: K times M rightarrow C listed in (a) - (c), answer the following questions: Is e an encryption function? If e is an encryption function, what is its associated decryption function d? If e is not an encryption function, can you make it into an encryption function by using some smaller, yet reasonably large, set of keys? e_k(m) k - m (mod N). e_k(m) k middot m (mod N). e_k(m) (k + m)^2 (mod N)