Hello, this is a Discrete Structure II problem, please help.

Decrypt the messageXJEOSXTQXTwhich was encrypted using the affine cipher:

f(p) = (17p + 3) mod 26

Alphabet:A = 0, B = 1, ..., Z = 25

Message: ?

PS: I got EATHXEOPEO and professor said it's not correct. He said: EATHXEOPEO is not correct.You seem to be using the encryption function to decrypt.You first need to figure out inverse of the encryption function.

Please help me in this problem. Thanks.