⇒
$| x + 9 | . 2 = 10$
$⇒ | x + 9 | = 10 ÷ 2$
$⇒ | x + 9 | = 5$
⇒ \(\left[ \begin{array}{l}x+9=5\\x+9=-5\end{array} \right.\)
⇒ \(\left[ \begin{array}{l}x=5-9\\x=-5-9\end{array} \right.\)
⇒ \(\left[ \begin{array}{l}x=-4\\x=-14\end{array} \right.\)
Vậy $x ∈ \{ -4 ; - 14 \}$