Figured someone would have helped you out by now. Anyway, the nut that holds on the sprocket has a washer that is actually bent over to keep it from coming off. Straighten out the washer. Then get a socket to fit. Try to get a six point. You will also need a lot of leverage. An 18" breaker bar works or you can use a cheater, a piece of pipe over the handle of another tool. Put it in gear and hold the rear brake and it should come off. It help to have two people.
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