Minrich my xinitrc file is also under /etc/X11/xinit/ directory
but my computer is 32bit.

but MTK358 asks for .xinitrc for users home directory
and at my home directory there is no .xinitrc file.

#!/bin/bash
# $Xorg: xinitrc.cpp,v 1.3 2000/08/17 19:54:30 cpqbld Exp $

# /etc/X11/xinit/xinitrc
#
# global xinitrc file, used by all X sessions started by xinit (startx)

# invoke global X session script
. /etc/X11/Xsession

I presume that, after reading the xrandr manpage, you have tried either:

or

xrandr --output VGA --mode 1680*1050
xrandr: cannot find mode 1680*1050

xrandr works for resolutions under 1280*800 but not
for others like 1680*1050 or 1440*900

Use an "x" instead of an "*".

Yeap you are right sorry to use * .

By the way thank you both guys.

The solution is
We have to add 1680*1050 with addmode option.
and then with --mode option we have to select 1680*1050.

Voila it works !