[tex]\displaystyle 9^x+3^x-12=0\\ \\
(3^x)^2+3^x-12=0\\ \\ \\
t=3^x\Rightarrow t^2+t-12=0\\ \\
\left \{ {{t_1+t_2=-1} \atop {t_1\times t_2=-12}} \right. \Leftrightarrow \left \{ {{t_1=3} \atop {t_2=-4}} \right. \\ \\ \\
Astfel:\\ \\
\left \{ {{3^x=3} \atop {3^x=-4}} \right. \Leftrightarrow \left \{ {{x=1} \atop {imposibil}} \right. \\ \\ \\
[/tex]