Is the 4th term is -15 and the 5th term is 45, then you would set it up like a5=a4(r)^5-4 45=-15(r)^1 Then you would get -3=r so the common ratio is -3, then you have to solve for a1 which would copy the previous format like -15=a1(-3)^3 which would be (5/9). The formula for a geometric sequence is an=a1(r)^n-1 The formula for a limited geometric sequence is Sn=a1((1-r^n)/(1-r)) (5/8)((1-(-3)^7)/(1-(-3)) (5/9)(16385/5) 2735/9=303 8/9 So D!