When n is even, (-1)^3n will be positive, and when n is odd, (-1)^3n will be negative.
So we can split the sum into two parts: one for even values of n and one for odd values of n.
For even n:
(-1)^3n = 1
So the sum becomes:
1 * 3n = 3n
For odd n:
(-1)^3n =
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