College Algebra (6th Edition)

$-258$
We can see that the ratio of subsequent terms is $5$, thus $r=-2$. The sum of the first $n$ terms can be obtained by multiplying the first term($a_1$) by the difference of $1$ and the common ratio ($r$) on the power of $n$ divided by the difference of $1$ and the common ratio ($r$). So $S_n=\frac{a_1(1-r^n)}{1-r}$. Hence here $S_{7}=\dfrac{-6(1-(-2)^{7})}{1-(-2)}=.258$.