El Gamal encryption:

In the El Gamal cryptosystem, Alice and Bob use p = 17 and g = 3. Alice chooses her secret to be a = 6, so g^a mod p = 3^6 mod 17 = 15. Alice publishes (p,g,g^a mod p) = (17,3,15). Bob sends the ciphertex (7, 6). That is, he chooses a secret 1 b p 1 and tells Alice that g^b 7 (mod p) and (g^a)^b m 6 (mod p). Determine the plaintext m.