Question: How do I put _X in a command

Not sure if this is the right place to ask this (if not, where is?)

Suppose I ask a question and get an aswer with a variable such as F=_X+5_X^2+7 and want to sort it so as to get 


If I try sort(F,_X) it will read the underscore as starting a subscript... I can just highlight copy and paste an _X from the previous answer, but that is silly.

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