Answer
$-258$
Work Step by Step
The general formula to find the sum of a finite geometric sequence is given as: $ S_{n}=\dfrac{a_1(1-r^n)}{1-r}$
Here, $ a_1=-6$ and $ r=-2$
Plug $ n=6$
$ S_{n}=\dfrac{a_1(1-r^n)}{1-r}$
Now, $ S_{7}=\dfrac{-6 \times [1-(-2)^7]}{1+2}=-258$