Python decrypting ciphers!

Write a function called decryptstring that accepts a ciphertext byte string, a multiplier (m), and shift amount (k). It returns an array of integers containing the decrypted values.

it needs to be able to decrypt anything put into this function:

def linearstring(plaintext, m, k): return [linear(p, m, k) for p in plaintext]

def decryptstring( ciphertext, m, k):

these need to work:

decryptstring(b"s\x04''6\xab\xaa\x18\x04EE\xf0", 5, 11) should return [72, 101, 108, 108, 111, 32, 83, 105, 101, 114, 114, 97]

decryptstring(b'AB', 13, 50) should return [139, 80]