For a geometric progression, each term is multiplied by the same common ratio. Since this question asks for a sum, use the finite GP sum formula rather than adding terms manually. For r≠1, S_n=a(r^n-1)/(r-1). Hence S_8=-3((2)^8-1)/(2-1)=-765. Keep the sign of the first term and the ratio carefully, because they control whether the sum is positive or negative.