What is the third term in the binomial expansion of (3x + y3)4?

In a binomial expansion, the third term of (a + b)^4 uses the squared powers of both parts: C(4,2)a²b². Here, a = 3x and b = y³, so the term is 6(3x)²(y³)² = 6 · 9x² · y⁶ = 54x²y⁶. The third term is 54x²y⁶, choice D.

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