how do you find fist term, common difference,and Sn

Question

Find the first term, the common difference, and t for a series having S_n = 7n −2n^2 .

Answer 1

S1 = 7-2 = 5 is the first term

S2 = 14 -5 = 10 is the 2nd term

S3 = 21-18 = 3 is the 3rd term

S4 = 16 - 32 = -16 is the 4th term

Its a quadratic function, so no common difference, as

you can see