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b in (-oo:+oo)
(-4/7)*b-(7*b)+3/8 = 5109/56 // - 5109/56
(-4/7)*b-(7*b)+3/8-(5109/56) = 0
(-4/7)*b-7*b+3/8-5109/56 = 0
-53/7*b-636/7 = 0 // + 636/7
-53/7*b = 636/7 // : -53/7
b = 636/7/(-53/7)
b = -12
b = -12
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