Decryption of a basic encryption

[BENGREEN1]

excel help.jpg
hello, (please view excel help.jpg first and ignore excel.jpg or none of this will make sence) sorry to ask but i am in need of help for this decryption question. The question states:
A)Encryption can be made more secure by making the encryption algorithm more complex. One way to do this is to use two keys. This is shown in the spreadsheet fig. In this case the letter "A" is coded as 1, B is 2 etc. A secert code message is one key and the letter displacement (offset) is the other. In this case the seceret message is OCR GCSES ROCK QL, and the displacement is 4
B)Develop a spreadsheet to decypt a message encrypted using the double key method described above

I am able to complete question A easily and this is done using a mod function on the (Number+displacement+code letter value)+1, however the second question also states i can only see the things as shown in green aswell as the ascii code because you can figure that out with =CODE function.
So i need help from anyone please no matter how small to decode it only with the things in green. Also the decoding has to be variable according to the changes.
Any help is greatly appreaciated as i need the help for revision.
Regards