a, $|9+x|=2x$
⇔ \(\left[ \begin{array}{l}9+x=2x\\9+x=-2x\end{array} \right.\)
⇔ \(\left[ \begin{array}{l}x=9\\x=-3\end{array} \right.\)
b, $|x+6|=2x+9$
⇔ \(\left[ \begin{array}{l}x+6=2x+9\\x+6=-2x-9\end{array} \right.\)
⇔ \(\left[ \begin{array}{l}x=-3\\x=-5\end{array} \right.\)