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b in (-oo:+oo)
2*b+1/3 = 19/21 // - 19/21
2*b+1/3-(19/21) = 0
2*b+1/3-19/21 = 0
2*b-4/7 = 0 // + 4/7
2*b = 4/7 // : 2
b = 4/7/2
b = 2/7
b = 2/7
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