Answer
$17936135$
Work Step by Step
We can see that the ratio of subsequent terms is $-3$, thus $r=-3$.
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 $S_{15}=\frac{5(1-(-3)^{15})}{1-(-3)}=17936135$.
