Đặt $t=3^{2x+4}$.
Phương trình đã cho trở thành:
$t^2-12t+27=0$
$\Leftrightarrow \left[\begin{array}{l}t=3\\t=9\end{array}\right.$
$\Leftrightarrow \left[\begin{array}{l}3^{2x+4}=3\\3^{2x+4}=9\end{array}\right.$
$\Leftrightarrow \left[\begin{array}{l}2x+4=1\\2x+4=2\end{array}\right.$
$\Leftrightarrow \left[\begin{array}{l}x=-\dfrac{3}{2}\\x=-1\end{array}\right.$