What is the first term in the sequence for which d = -2 and a7 = 3?

For an arithmetic sequence, the seventh term is six common differences after the first term: a7 = a1 + 6d. Substituting a7 = 3 and d = -2 gives 3 = a1 + 6(-2) = a1 - 12, so a1 = 15. The first term is 15, choice C.

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