Use the formula \displaystyle S_n=a_1\cdot \frac{1-r^{n}}{1-r}S
n
​
=a
1
​
⋅
1−r
1−r
n

​
​ to evaluate the following sum.
\displaystyle \sum_{k=1}^{7}\left(-10 \left(3\right)^{k}\right)
k=1
∑
7
​
(−10(3)
k
)