To shut down a non-responsive app:
Use Alt+F2 to open a dialog box and type xkill
Use the mouse to drop the x on the application.
To close an app without using the mouse:
Alt+F2, type gnome-terminal
run ~$ top
Use sudo kill -9 “PID”
to kill the process using the most memory, where “PID” is the PID number.
To shut down and restart a non-responsive system:
Hold down the Alt & SysReq/PrntScr keys while yo type (slowly) these letters:
See this post for an explanation.