Answer :
[tex]3^x-3^{x-1}=18\\\\3^x-3^x\cdot3^{-1}=18\\\\3^x-\frac{1}{3}\cdot3^x=18\\\\\frac{2}{3}\cdot3^x=18\ \ \ /\cdot\frac{3}{2}\\\\3^x=27\\\\3^x=3^3\iff x=3\\------------------\\4^x=4^3=64\leftarrow answer[/tex]
[tex] 3^x - 3^{x-1} = 18\\
3^x-3^x\cdot3^{-1}=18\\
3^x(1-\frac{1}{3})=18\\
3^x\cdot\frac{2}{3}=18\\
3^x=27\\
x=3\\\\
4^x=4^3=64
[/tex]