You have:
S3 = 1/2 (S7-S3)
3/2 (24+2d) = 1/2 (7/2 (24+6d) - 3/2 (24+2d))
d = 2
AP = 12,14,16, 18,20,22,24
I'll let you verify the result.
Find an A.P. such that the sum of the first three terms is one half the sum of the next four terms, the first term being 12.
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