What is the sum of the geometric sequence - 3, 18, - 108, ... 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