Answer:
a(n) = 9·6^(n-1)
Explanation:
The general term a(n) of a geometric sequence with first term a(1) and common ratio r is given by ...
a(n) = a(1)·r^(n-1)
Your sequence has first term 9 and common ratio 54/9 = 6, so the function describing it is ...
a(n) = 9·6^(n-1)