1. In my Ubuntu 12.04, in keyboard settings, I didn't find a shortcut key for restarting X, so in "Customer Shorcut", I set up Ctrl+Alt+Backspace for command sudo restart lightdm. But after that the shortcut doesn't work. Is it because it requires root privilege?
2. Also I have a SysRq key on my keyboard, which I think to be the "Magic SysReq Key". My SysRq key is shared with PrtSc key (for screen shoot), and is in blue which means I have to press Fn key at the same time to invoke SysRq instead of PrtSC. But every time I press Fn+SysRq, it always shoots a photo of the screen, same as just hitting PrtSc i.e. without hitting Fn. I wonder how to use the Magic SysReq Key? Does it mean the shortcut has not been linked to any command that is supposed for Magic SysReq Key yet?

PS: My laptop is Lenovo T400 and OS is Ubuntu 12.04.

Thanks!