As far as I know, I can easily transform $ - (21x -7) $ into $ 7 - 21x $
But can I do the same with a module?
Would $ - |21x -7| $ also easily transform into $ 7 - 21x $? Or only transforming into $ -|7 - 21x| $ would be correct?
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The answer to your first question is No, since $-|21x-7|\le 0$, whereas $7-21x\le 0$ if & only if $x\ge \dfrac13$. – Bernard Sep 29 '21 at 12:30
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The second transformation is correct, since $|Q|=|-Q|$ for all $Q$. – Gerry Myerson Sep 29 '21 at 13:28