Mark M. answered 06/25/18
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Retired math prof. Very extensive Precalculus tutoring experience.
a4 = 4 Let d = common difference.
First 7 terms: 4-3d, 4-2d, 4-d, 4, 4+d, 4+2d, 4+3d
Sum of first 7 terms = 4(7) = 28