What is the sum of the geometric sequence -3, 18, - if there are 7 terms?

The sum of the geometric series when the common ratio is less than 1 will be given by:
Sn=a(1-r^n)/(1-r)
from our sequence;
a=-3
r=-108/18=-6
n=7
thus the sum of the 7 terms will be:
S7=-3(1-(-6)^7)/(1-(-6))
S7=-3(1+279936)/(1+6)
S7=-3(279937)/7
S7=-119,973
the answer is -119,973