1. rm ~/.config/monitors.xml && gnome-control-center
2. setup displays
3. sudo apt-get install libxml2-utils
4. sudo wget -O /usr/local/sbin/update-monitor-position http://www.calgorithms.com/assets/files/update-monitor-position && sudo chmod +x /usr/local/sbin/update-monitor-position
5. System - Startup Applications tool > update-monitor-position