What is the explicit rule for the arithmetic sequence 3, 9, 15, 21, 27, . . . ?

The sequence increases by 6 each time, so its common difference is 6. Using the arithmetic-sequence rule aₙ = a₁ + (n − 1)d, with first term 3 and common difference 6, gives aₙ = 3 + (n − 1)6. This is choice A.

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