Answer
$-510$.
Work Step by Step
The sum of a geometric sequence until $n$ can be obtained by the formula $S_n=a_1(\frac{1-r^n}{1-r})$ where $a_1$ is the first term and $r$ is the common ratio. Hence here the sum is: $S_8=6(\frac{1-(-2)^8}{1-(-2)})=-510$.