Explanation:
the formula for the sum of the first n terms of a geometric sequence is
Sn = s1(1 - r^n)/(1 - r)
with r being the common ratio and s1 is the first term.
so,
S15 = 7×(1 - (-3)¹⁵)/(1 - -3) = 7×(1 - -14,348,907)/4 =
= 7×14,348,908/4 = 7×3,587,227 = 25,110,589