here, first term(a) = -8
Last term(tn) = 7
Common difference (d) = -5-(-8) =-5+8 =3
We know,
tn = a+ (n-1)d
7 = -8 + (n-1)3
15 = (n-1)3
n-1 = 15/3 =5
n=5+1
n=6
Therefore, Sum of the series(Sn) = n/2{2a +(n-1)d}
= 6/2 {-8 *2 + (6-1)*3}
= 3 *{-16+15}
=3*-1
= -3
