a, $| x+3|-7= 3x$
⇔ $| x+3|= 3x+7$
⇔ \(\left[ \begin{array}{l}x+3=3x+7\\x+3= -3x-7\end{array} \right.\)
⇔ \(\left[ \begin{array}{l}x=2\\x=-2,5\end{array} \right.\)
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b, $| x-5|-3x= 19$
⇔ $|x-5|= 19+3x$
⇔ \(\left[ \begin{array}{l}x-5=19+3x\\x-5= -19-3x\end{array} \right.\)
⇔ \(\left[ \begin{array}{l}x=-12\\x=-3,5\end{array} \right.\)
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c, $| 3-x|-3x= 11$
⇔ $|3-x|= 11+3x$
⇔ \(\left[ \begin{array}{l}3-x= 11+3x\\3-x= -11-3x\end{array} \right.\)
⇔ \(\left[ \begin{array}{l}x=-2\\x=-7\end{array} \right.\)
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