the problem is in geometric progression since you are given 'r' or the common ratio.
You have the formula for the sum of geometric progression as:
[tex]S_n = \frac{A_1(1-r^n)}{(1-r)} \\ you~are~given~the~following~data: \\ S_7 = 7651 \\ r=3 \\ n=7 \\ using ~the ~formula~you~can~find~A_1 \\ 7651 = \frac{A_1(1-3^7)}{1-3} \\ 7651 = \frac{A_1(1-2187)}{-2} \\ 7651(-2) = -2186A_1 \\ -15,302 = -2186A_1 \\ A_1 = 7[/tex]
