At some time in the past you checked "Encrypt Backup". When you did you were prompted to enter a passcode, then enter it a 2nd time for confirmation. This is the passcode you need. If you have a business MS Exchange email account your account administrator may have forced backup encryption, but you still would have had to enter the passcode twice. The passcode is only required the first time you make an encrypted backup; after that the original passcode will be used unless you explicitly change it. So if you set it 2 years ago that passcode hasn't changed.
On the plus side you get unlimited guesses; try unchecking Encrypt Backup and you will be prompted immediately, which speeds up the process of trying old passcodes. I've compiled a list from various message threads of passcodes that were guessed successfully. You can find them in the attached document: