`|x+9|=|2x+3|`
`=>`\(\left[ \begin{array}{l}x+9=2x+3 \rm \ ( khi \ x\ge 0)\\x+9=-2x-3\rm \ (\ khi \ x <0)\end{array} \right.\)
`<=>` \(\left[ \begin{array}{l}-x=-6\\3x=-12\end{array} \right.\)
`<=>`\(\left[ \begin{array}{l}x=6\ (tm) \\x=-4\ (tm)\end{array} \right.\)
Vậy `S={6; -4}`