$$a\cdot 3^{x-1}+b\cdot 3^x=c\\ a\cdot 3^{x}\cdot 3^{-1}+b\cdot 3^x=c$$
\(3^x\) ausklammern:
$$3^{x}\left( \frac{a}{3}+b\right)=c \\3^x=\frac{c}{\left( \frac{a}{3}+b\right)}$$
Logarithmus anwenden:
$$x\cdot ln(3)=ln\left(\frac{c}{\left( \frac{a}{3}+b\right)}\right)\\ x=\frac{ln\left(\frac{c}{\left( \frac{a}{3}+b\right)}\right)}{ln(3)}$$