Question 1053000
S = (n/2)(2a + (n-1)d)
-120 = 15/2 (2a + 14(-3))
-16 = 2a - 42
26 = 2a
a1 = 13
{{{a[n]=a[1]+(n-1)*d}}}
a15 = 13 + (-42)
a15 = -29
Checking

{{{S[n]=(n(a[1]+a[n]))/2}}}
-120 = 15(13-29)/2   = 15(-16)/2 = -120 CHECKS