tolong dijawab ya kak, please!!!!

8. a = 201
Un = 399
kelipatan (3) selisih = b
Un = a + (n-1)b
Un = 201 + (n-1) 3
399 = 201 + (3n-3)
198 = 3n - 3
201 = 3n
n = 67
SN = n(Un+a) ÷ 2
67(399+201) ÷ 2 = 20.100