`a,2(x-6)+30=-42`
`=> 2(x-6)=-42-30`
`=> 2(x-6)=-72`
`=> x-6=-72÷2`
`=> x-6=-36`
`=> x=-36+6`
`=> x=-30`
Vậy `x=-30`
`b,|x-5|+12=30`
`=> |x-5|=30-12`
`=> |x-5|=18`
\(⇒\left[ \begin{array}{l}x-5=18\\x-5=-18\end{array} \right.\)
\(⇒\left[ \begin{array}{l}x=18+5\\x=-18+5\end{array} \right.\)
\(⇒\left[ \begin{array}{l}x=23\\x=-13\end{array} \right.\)
Vậy `x={23;-13}`
`c,(x-2).(2x+4)=0`
\(⇒\left[ \begin{array}{l}x-2=0\\2x+4=0\end{array} \right.\)
\(⇒\left[ \begin{array}{l}x=0+2\\2x=0-4\end{array} \right.\)
\(⇒\left[ \begin{array}{l}x=2\\2x=-4\end{array} \right.\)
\(⇒\left[ \begin{array}{l}x=2\\x=-4÷2\end{array} \right.\)
\(⇒\left[ \begin{array}{l}x=2\\x=-2\end{array} \right.\)
Vậy `x=±2`
