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b in (-oo:+oo)
2/3-(3*b) = -40/21 // + -40/21
2/3-(3*b)-(-40/21) = 0
2/3-3*b+40/21 = 0
18/7-3*b = 0 // - 18/7
-3*b = -18/7 // : -3
b = -18/7/(-3)
b = 6/7
b = 6/7
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