Answer
$17936135$
Work Step by Step
Since, we have $S_n=\dfrac{a_1(1-r^{n})}{1-r}$
Here, $n=15,a_1=5,r=-3$
Thus,
$S_{15}=\dfrac{5(1-(-3)^{15})}{1-(-3)}=17936135$
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