$)$Alice uses a public key cryptosystem with

a public key $(33,3).$ She encodes letters by assigning numerical values to them based on their

positions in the alphabet, where A is assigned the value $1,$ B is assigned $2,$ and so on$,$ up to Z

being assigned the value $26.$

$($a$)$ Bob wants to send Alice the message KEY. Determine the ciphertext for his message.

$($b$)$ Determine Alice$$s private key and decrypt the message $18215.$