Check your graphic card details. You may check this at the terminal:
lspci -vvnn | grep VGA
or you may enter
lspci -nn | grep VGA
XXX@XXX-desktop:~$ lspci -nn | grep VGA
01:05.0 VGA compatible controller : Advanced Micro Devices, Inc. [AMD/ATI] RS780 [Radeon HD 3200] [1002:9610]
02:00.0 VGA compatible controller : Advanced Micro Devices, Inc. [AMD/ATI] RV730 PRO [Radeon HD 4650] [1002:9498]
Xrandr Graphical Front End GUI
ARandR is designed to provide a simple visual front end for XRandR.
Relative monitor positions are shown graphically and can be changed in a drag-and-drop way.
LXRandR is the standard screen manager of LXDE. It manages screen resolution and external monitors.
You can plug in another screen into LXDE or choose to use a big screen projector.
Local screen and extenal screen can be used at the same time. LXRandR configures the screen solution automatically.