Affine Cipher Cryptography

Its kinda like subtitution cipher, where the plain text are converted to its ascii/utf counterpart (or simply 1 to 26), then apply a mathematical formula to each numbers, and convert it back to letters. ex. plaintext: HALLO formula: 3x+1 to number: 8 1 12 12 15 apply formula: 25 4 37 37 45 mod 26: 25 4 11 11 19 ciphertext: Y D K K S