The coefficient of the x^3 term in the expansion of (3x + a)^4 is 216. Find the value of a.
From the binomial theorem we know that the x^3 term in the expansion of the above expression must satisfy,
4C3 * (3x)^3 * a = 216x^3.
Hence, after multiplying out we must have,
108a * x^3 = 216x^3
and therefore the value of a must be 2.
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