Answer: common difference is 14
Explanation:
a,a+d,a+4d
7,7+d,7+4d
(7+d)/7=(7+4d)/(7+d)
(7+d)(7+d)=7(7+4d)
49+14d+d^2=49+28d
d^2-14d=0
d(d-14)=0
d=0 or d-14=0
d=14
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