Question

 $3^{x}- 3^{x-2}+ 3^{x+1}=105$

Answer 1

$3^x-3^{x-2}+3^{x+1}=105 \\ \\ 3^x-\frac{3^x}{9}+3.3^x=105 \\ \\ Substituicao: \ \ \boxed{3^x=y} \\ \\ y-\frac{y}{9}+3y=105 \\ \\ 4y-\frac{y}{9}=105 \\ \\ 36y-y=945 \\ \\ 35y=945 \\ \\ y=27 \\ \\ 3^x=27 \\ \\ \boxed{3^x=3^3 \rightarrow x=3}$