help! 71+64+57+...+x=396 a.-3 b.-2 c.-1 d.1 e.2 pake cara ya.terimakasih:)

U1 = 71 = a
U2 = 64 = a + b ---> 71 + b = 64 ---> b = (-7)
U3 = 57
Sn =396
Un = x = a + (n - 1)b = 71 + (n - 1)(-7) = 71 - 7n + 7
Un = x = 78 - 7n

Sn = n/2 (a + Un)
396 = n/2 ( 71 + 78 - 7n)
792 = n (149 - 7n)
792 = 149n - 7n²
7n² - 149n + 792 = 0
(n - 11) (7n -72) = 0
n = 11 atau n = 72/7
ambil nilai n  yang bukan pecahan
n = 11
maka nilai X = 78 - 7(11) = 1