|9+x|=2x

* Hướng dẫn giải

$\begin{array}{l}\left|9+x\right|=2x\left(10\right)\\ +)x\ge -9\Rightarrow \left(10\right)\iff 9+x=2x\iff x=9(tm)\\ +)x<-9\Rightarrow \left(10\right)\iff -9-x=2x\iff x=-3(ktm)S=\left\{9\right\}\end{array}$