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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.

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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.

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To shut down and restart a non-responsive system:

Hold down the Alt & SysReq/PrntScr keys while yo type (slowly) these letters:

r-e-i-s-u-b

See this post for an explanation.