My sewer line backs up when I use the wash machine. I took the clean out cap off, so it doesn't back up in the house. However it does backup at the cleanup. How do I stop this washing machine clog.

Its not a washing machine clog it's your main drain line the washer runs into. Rod down that cleanout towards the street moving away from the house. The clog can be far downstream since most clogs in a main are right where the drain connects to te city sewer, you are on city sewer, right?