Find the sum of the geometric sum: 12 36 108 78,732

Ok, geometric
seems to be multiply by 3 each time
hmm, so
the sum of a geometric sequence where a1 is the first term, n is which term and r=common ratio

Sn=a1(1-r^n)/(1-r)
so
we need to find which term
so
an=a1(r)^(n-1)
a1=first term=12
and common ratio is 3=r
and the nth term is 78732
78732=12(3)^(n-1)
78732=12(1/3)(3^n)
78732=4(3^n)
divide both sides by 4
19683=3^n
use math to solve and get
9=n
so that was the 9th term

a1=12
r=3
n=9

S9=12(1-3^9)/(1-3)
S9=12(1-19683)/(-2)
S9=-6(-19682)
S9=118092

the sum is 118092