We must find the common ratio first...
Since a(n)=ar^(n-1)
-3359232=12r^7
-279936=r^7
-279936^(1/7)=r
-6=r
So our geometric sequence is:
a(n)=12(-6)^(n-1)
And the sum of a geometric sequence is:
s(n)=a(1-r^n)/(1-r), since a=12, r=-6, and n=8 we have:
s(8)=12(1-(-6)^8)/(1--6)
s(8)=12(1-(-6)^8)/7
s(8)= -2,879,340
