\(\left(I\right)\left\{{}\begin{matrix}x< 0\\-12x+4-x+2x-10-7x+21=12=>-18x=-3\left(l\right)\end{matrix}\right.\)\(\left(II\right)\left\{{}\begin{matrix}0\le x< \dfrac{1}{3}\\-12x+4+x+2x-10-7x+21=12\Rightarrow-16x=-3x=\dfrac{3}{16}\left(n\right)\end{matrix}\right.\)\(\left(III\right)\left\{{}\begin{matrix}\dfrac{1}{3}\le x< 3\\12x-4+x+2x-10-7x+21=12\Rightarrow8x=5\Rightarrow x=\dfrac{5}{8}\left(n\right)\end{matrix}\right.\)
\(\left(IV\right)\left\{{}\begin{matrix}3\le x< 5\\12x-4+x+2x-10+7x-21=12\Rightarrow20x=47\Rightarrow x=\dfrac{47}{20}\left(l\right)\end{matrix}\right.\)
\(\left(V\right)\left\{{}\begin{matrix}x\ge5\\12x-4+x-2x+10+7x-21=12\Rightarrow18x=27\Rightarrow x=\dfrac{27}{18}\left(l\right)\end{matrix}\right.\)
\(\left[{}\begin{matrix}x=\dfrac{3}{16}\\x=\dfrac{5}{8}\end{matrix}\right.\)
