question is simple how can I create shot down short cut on keyboard so when I press that buttons computer will shot down
There is already a shortcut for this. http://en.wikipedia.org/wiki/Magic_SysRq_key
I assume your case it's ctrl + alt + printscr + o. But look it up in the table provided there.
you can also create your own shortcuts by using xbindkeys, see this tutorial: http://www.butlerpc.net/blog/2011/01/using-xbindkeys-on-ubuntu-linux-to-remap-key-commands/