`||x+3|-10|=20`
$\Leftrightarrow$ $\left[\begin{matrix} |x+3|-10=20\\ |x+3|-10=-20\end{matrix}\right.$
$\Leftrightarrow$ $\left[\begin{matrix} |x+3|=20+10\\ |x+3|=-20+10\end{matrix}\right.$
$\Leftrightarrow$ $\left[\begin{matrix} |x+3|=30\\ |x+3|=-10(loại)\end{matrix}\right.$
$\Leftrightarrow$ `|x + 3| = 30`
$\Leftrightarrow$ $\left[\begin{matrix} x+3=30\\ x+3=-30\end{matrix}\right.$
$\Leftrightarrow$ $\left[\begin{matrix} x=30-3\\ x=-30-3\end{matrix}\right.$
$\Leftrightarrow$ $\left[\begin{matrix} x=27\\ x=-33\end{matrix}\right.$
$\text{Vậy}$ `x \in {27 ; -33}`
