Answer question completely and carefully with full working out for a thumbs up, it is on cryptography.

Exercise 3: Alice wants to use the Hill cipher with block size l = 3 on an alphabet of 26 characters. She chooses the key K = [ 9 21 0 1 4 16 0 1 16). 11 Prove that K is a valid encryption key. Use this key to encrypt the plaintext 'WUT