The sum of n terms is given from the first term and the common ratio by
[tex]S_{n}=a_{1}\dfrac{r^{n}-1}{r-1}[/tex]
For your given values of a1=-11, r=-4, n=8, the sum of 8 terms is
[tex]S_{8}=(-11)\dfrac{(-4)^{8}-1}{-4-1}=(-11)\dfrac{65535}{-5}\\\\=144177[/tex]
