Consider an affine cipher on Z$28$ that work over $$ABCD$....$XYZ$?!$ where the numerical

equivalent of $?$ and $!$ are $26$ and $27,$ respectively. The encryption function to compute

ciphertext C is given by C $=$ a$.$P $+$ b mod $28$ where P is the plaintext, a $=14,$ and b $=3.$

a$)$ Is $14$ a valid choice for $$a$?$ Why?

b$)$ Is $3$ a valid choice for $$b$?$ Why?

c$)$ Encrypt the plaintext $$AFFINE$.$