Below is a possible solution to the Challenge 53 – Cryptography Challenge 4 – Caesar's Shift Cipher Decrypter.
Put following formula and drag down
=IFERROR(CHAR(IF(CODE(LOWER(G2))-$B$3<97,CODE(G2)-$B$3+26,CODE(G2)-$B$3)),"")
The Excel file containing the solution can be downloaded from Solution – Challenge 53 – Decryption – Caesar's Shift Cipher