(S_9=171), (a_8=31)

[ \dfrac{9}{2}[2a+8d]=171\implies 9(a+4d)=171\implies a+4d=19\tag{1} ] [ a+7d=31\tag{2} ]

(2)-(1): (3d=12\implies d=4), (a=19-16=3)

Series: (3,7,11,15,\ldots)

Answer: (3,7,11,15,\ldots)