|x+3|+|9-x|=15
\(\Leftrightarrow\left\{{}\begin{matrix}x+3+9-x=15\\-x+3+9-x=15\\x-3-9+x=15\\-x-3-9+x=15\end{matrix}\right.\)
\(\Leftrightarrow\)\(\left\{{}\begin{matrix}12=15\left(ktm\right)\\-2x+12=15\\2x-12=15\\-12=15\left(ktm\right)\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}x=-1,5\\x=13,5\end{matrix}\right.\)
Vậy.....