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The correct sequence is 6, 18, 54, 162. Let's test it with the formula:
a(n) = an-3a(n-1)
a(2) = 2-3a(1) = 2 - 3*6 = -16
a(3) = 3-3a(2) = 3 - 3*18 = -51
a(4) = 4-3a(3) = 4 - 3*54 = -156
Therefore, the first four terms of this sequence are 6, 18, 54, 162.