A1=7,an=21,Sn=2,n=? d=?

n = (an - a1) / d + 1 n = (21 - 7) / 2 + 1 n = 14 / 2 + 1 n = 7 + 1 n = 8

d = (an - a1) / (n - 1) d = (21 - 7) / (8 - 1) d = 14 / 7 d = 2