Find the sum of the first 8 terms in the sequence?

We need to find the 8 th term nth term is given as an=a1+(n−1)d n=8, d =-4 (common difference) a8=a1+(8−1)×d a8=1+7×(−4)

I’m guessing that the answer is c, but i’m not sure which grid is being talked about. since 5x5= 25 and 3x3= 9 (which there are no answers for). a is not right because while the ratio is the same, 6/25= 0.24 which is equal to 24% not 30%.