well i have to figure that out. what i have done is i have made a file with a compact name like "res" and pasted following code in this file
-----------------------------------------
xrandr --newmode "1368x768_60.00" 85.86 1368 1440 1584 1800 768 769 772 795 -HSync +Vsync
xrandr --addmode VGA1 1368x768_60.00
xrandr --output VGA1 --mode 1368x768_60.00
------------------------------------------
saved this file in /home/myname
now everytime i start my computer i open terminal --> type ./res
and theres my required resolution.
i am not making and editing xorg.conf file because i did it once and my computer was stuck on boot up.
so am happy with this procedure :)

