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b in (-oo:+oo)
51/56 = b+5/8 // - b+5/8
51/56-b-(5/8) = 0
51/56-b-5/8 = 0
2/7-b = 0 // - 2/7
-b = -2/7 // * -1
b = 2/7
b = 2/7
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