Sn = (a₁+an) x n /2 = 210 => n = (210x2)/(a₁+an) = 420/42 =10
n = 10 termeni
an = a₁+ r(n-1) => r = (an-a₁)/(n-1) = (39-3)/9 = 36/9 = 4
r = 4
