In DASHCAT 4, quant ques no 67, we had Sn = 2n^3 + n^2 + 3n + 1. We also have tx = 84. In the solution, we did we got the value of x by tx = Sx - S(x-1). I don't understand why the answer is different when I've calculated the value of S(1) = t(1) = 7, and S(2) = 27. After it S(2) - S(1) = t(2) = 20. So I got d = 13. Tell me where I am wrong in this approach?