The reason you bash can't find the command xf86config is because you're running Slackware 10, which uses X11 6.7 instead of XFree86. The command you need is xorgconfig.
Also, the file you need to edit is /etc/X11/xorg.conf. Not /etc/X11/XF86Config.
To edit a system file, you need to open it in an editor, with root privelege.
Eg. Open a terminal and enter the following.
Code:
james@atlantis:~$ su
Password:
root@atlantis:/home/james# mcedit /etc/X11/xorg.conf
su=switch user. If su if followed with no user name, root is assumed.
Password:Put roots password here. The terminal now has root privelege.
mcedit /etc/X11/xorg.conf opens the file you need for this exercise, using Midnight Commander's editor.