In a long string of a ciphertext which was encrypted by an affinemap on a single letter message units in the $26-$letter alphabet,you observe that the most frequently occurring letters are Yand V$,$ in that order.Assuming that those ciphertext message units are theencryption of E and T respectively.

$1.$ What is the key a and b$?$

$2.$ Encipher the plaintext message $"$ YES".