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b in (-oo:+oo)
2*b+1/3 = 13/21 // - 13/21
2*b+1/3-(13/21) = 0
2*b+1/3-13/21 = 0
2*b-2/7 = 0 // + 2/7
2*b = 2/7 // : 2
b = 2/7/2
b = 1/7
b = 1/7
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