Sebastian writes the recursive formula f(x+1) = 4f(x) to represent a geometric sequence whose second term is 12.

The recursive rule multiplies each term by 4, so the sequence has common ratio 4. If the second term is 12, then the first term is 12 ÷ 4 = 3. Thus the explicit formula is f(x) = 3(4)^(x − 1), which is choice B.

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