Since it is looking for the sum at S3, replace n with 3 , 2, and 1 , solve the equation and add all of answers together when s = 3 the equation becomes 5(1/3)^3-1 5(1/3)^2 = 5/9
when s = 2 the equation becomes 5(1/3)^2-1 5(1/3)^1 = 5/3
when s = 1 the equation becomes 5(1/3)^1-1 5(1/3)^0 = 5
