Python decrypting ciphers

write a function called lineardecipher that accepts a ciphertext byte string, a multiplier (m), and a shift amount (k). It returns the corresponding plaintext byte string.

def lineardecipher( cipertext, m, k):

This needs to work:

lineardecipher(b"s\x04''6\xab\xaa\x18\x04EE\xf0", 5, 11) should return b'Hello Sierra'