Let $\backslash$delta represent some fixed letter of the alphabet. Della encrypts the plaintext

PAWNSHOP with the message key AB$\backslash$delta $.$ The ciphertext includes some repeated letters in the

form $\backslash$alpha $\backslash$beta $\backslash$beta $\backslash$alpha $($that is$,$ the $1$st letter is the same as the $8$th letter, and the $4$th letter is

as same as the $5$th letter$).$ Determine with justification all possible values of $\backslash$delta or justify

why none exist.