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204, 210, 216, ..., 390, 396.
U n = a + (n-1) b
396 = 204 + (n-1)6
396 = 204 + 6n - 6
396 = 198 + 6n
6n = 396 - 198 = 198
n = 198/6 = 33
S33 = 33/2 (204+396)
= 33/2 (600) = 9900