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d in (-oo:+oo)
7*d-(4*d)+1 = 28 // - 28
7*d-(4*d)-28+1 = 0
7*d-4*d-28+1 = 0
3*d-27 = 0 // + 27
3*d = 27 // : 3
d = 27/3
d = 9
d = 9
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