Question 315228
Problem is not clear, so two solution is possible.
Solution 1:

{{{4(3^n-1)=324}}}
{{{3^n-1=81}}}
{{{3^n=80}}}

{{{nlog(10,3)=log(10,80)}}}

{{{n=log(10,80)/log(10,3)}}}


Solution 2:

{{{4(3^(n-1))=324}}}
{{{3^(n-1)=81}}}
{{{3^(n-1)=3^4}}}
{{{n-1=4}}}
{{{n=5}}}