a1+a2+a3=8
a4+a5+a6=-4
a1q^3+a2q^3+a3q^3=-4
q^3(a1+a2+a3)=-4
q^3=-1/2
a13+a14+a15
=a1q^12+a2q^12+a3q^12
=q^12(a1+a2+a3)
=(q^3)^4*8
=1/2
s15=a1+a2+a3+.+a13+a14+a15
=a1+a2+a3+q^3(a1+a2+a3)+...+q^12(a1+a2+a3)
=(a1+a2+a3)(1+q^3+q^6+q^9+q^12)
=8*(1-1/2+1/4-1/8+1/16)
=8*11/16
=11/2