Answer
$-258$
Work Step by Step
Since, we have $S_n=\dfrac{a_1(1-r^{n})}{1-r}$
Here, $n=7,a_1=-6,r=-2$
Thus,
$S_{7}=\dfrac{-6[1-(-2)^{7}]}{1-(-2)}=\dfrac{-6}{3}(129)=-258$
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