[xrandr][ArchLinux] After startx then execute xrandr to extend my desktop

There two way to do this.

Way one :
1. Settings --> Session and Startup --> Application Autostart --> Add.
Name : Xrand
Description : extend desktop
Command : xrandr --output LVDS1 --mode 1280x800 --left-of VGA1
Click OK --> Close --> reboot.

Way two :
1. Touch a file call Xrand.desktop at following folder and add following text.
/root/.config/autostart/Xrand.desktop

[Desktop Entry]
Encoding=UTF-8
Version=0.9.4
Type=Application
Name=Xrand
Comment=extend desktop
Exec=xrandr --output LVDS1 --mode 1280x800 --left-of VGA1
OnlyShowIn=XFCE;
StartupNotify=false
Terminal=false
Hidden=false

2. save --> reboot
Reference:

• How to Start Applications Automatically as Linux Boots