How to log off facebook after it wasn’t logged out of a different computer?

So I didn’t sign out of my facebook while I was at my parents last and my brother found out and is now threatening to change things on my facebook. I didn’t ask my parents pc to remember my password or anything, but it’s still kept me logged on for some reason and I would like to know how I can log my self out of their computer without physically going onto it and logging out. I’ve already logged onto facebook since using other computers, but this obviously hasn’t worked.

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