My problem is solved. the procedure i did was
After entering root
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# xrandr -q
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i got
Screen 0: minimum 320 x 200, current 1024 x 768, maximum 8192 x 8192
VGA1 connected 1024x768+0+0 (normal left inverted right x axis y axis) 0mm x 0mm
1024x768 60.0*
800x600 60.3 56.2
848x480 60.0
640x480 59.9
HDMI1 disconnected (normal left inverted right x axis y axis)
DP1 disconnected (normal left inverted right x axis y axis)
HDMI2 disconnected (normal left inverted right x axis y axis)
DP2 disconnected (normal left inverted right x axis y axis)
then i entered my required resolution
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# gtf 1366 768 60
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this gave me output
# 1368x768 @ 60.00 Hz (GTF) hsync: 47.70 kHz; pclk: 85.86 MHz
Modeline "1368x768_60.00" 85.86 1368 1440 1584 1800 768 769 772 795 -HSync +Vsync
copy the above line and paste with the following command
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xrandr --newmode "1368x768_60.00" 85.86 1368 1440 1584 1800 768 769 772 795 -HSync +Vsync
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then
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# xrandr --addmode VGA1 1368x768_60.00
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then
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xrandr --output VGA1 --mode 1368x768_60.00
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problem solved. am happy