Affine Cipher.

Assume that a = 15 and b = 12 in affine cipher. What is the encryption of 8, 11, and 13 (e(8), e(11), e(13))? What is the decryption of 8, 11, and 13 (d(8), d(11), d(13))?

An obvious approach to increase the security of a symmetric algorithm is to apply the same cipher twice, i.e.: y = e_k2(e_k1(x))