I am using a computer equipped with the GeForce GT 640 card, featuring one VGA port, two DVI ports and one HDMI port. For a long time, I have been using a single monitor plugged into the VGA port and it has worked fine.
I recently got a second monitor, which I plugged into the DVI port. For various reasons, I would like to keep my original monitor (on the VGA port) as the primary monitor. When I booted linux, the new monitor on the DVI port had, however, become the primary monitor. I tried to change this by going to Systems->Settings->Settings->Monitors, but was met with the message "could not get screen information". Note that this is translated to English, and might not be completely accurate.
How might I address this problem? Also, if would be happy to learn how to change screen/monitor settings from the command line.
*Debian: version 8.5
*Gnome: GNOME Shell 3.14.4