According to the formula of A.P.:
a7 = a1 + (7 - 1)d and a4 = a1 + (4 - 1)d,
15 = a1 + 6d and 9 = a1 + 3d,
a1 = 15 - 6d = 9 - 3d,
3d = 6,
d = 2 is the common difference.
a1 = 9 - 3×2 = 3 is the first term.
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