1)split: 3 ´powered to n-1 is equal to 3 power to n multiplied by 3 powered to -1:
[tex]=15\cdot3^3\cdot3^{-1}[/tex]2) Flip: a negative expinent (x powered to -1) is equal to 1/x.
[tex]=15\cdot3^n\cdot\frac{1}{3}[/tex]3. Multiply: multiply 15 by 1/3
[tex]\begin{gathered} =\frac{15}{3}\cdot3^n \\ \\ =5\cdot3^n \end{gathered}[/tex]4. Replace: replace n by x
[tex]f(x)=5\cdot3^x[/tex]