Answer: a(n)=3^n as an infinite sequence
I do not know whether you've been introduced to series notation yet, though if you haven't, a(n)=3^n pertains to sequences specifically.
a(n) = 3^n = {3^1, 3^2, 3^3, 3^4, 3^5...}
Which equals 3^n = {3, 9, 27, 81, 243...}
