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168 = 84 + (n-1)7
168 - 84 = 7n-7
84 = 7n-7
84+7 = 7n
7n = 91
n = 91/7
n = 13
Sn = n/2 (a+Un)
Sn = 13/2 (84+168)
Sn = 13/2.252
Sn = 1638