The idea of Public Key Cryptography is to send messages in such a way that only the person who receives the messages can understand them. Even if everyone knows the method of encryption nobody except the receiver has all the information needed in order to decypher the message. To solve this problem you are in the position of Bob receiving a secret number from Alice who has sent him the coded message 180. Bob is the only person in the world who knows that he has to use 59 in order to find the secret number $M$ which Alice has sent to him, and which he knows to be between 0 and 390.
Bob has to find the number equivalent to $180^{59}\pmod {391}$ so can you do this for him?