The affine cipher is a combination of additive cipher and multiplicative cipher (Link for more info https://www.geeksforgeeks.org/traditional-symmetric-ciphers/)
C = [(P * a) + b] mod n # C = Chiper text , P = Plain text, n = 26 (length of alphabets)
P = [(C – b) * a^(-1) ] mod n # (a,b) two keys are used for Affine Cipher.
Commented Code for better understanding.