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b in (-oo:+oo)
-2*b = -4/7 // + -4/7
-2*b-(-4/7) = 0
4/7-2*b = 0
4/7-2*b = 0 // - 4/7
-2*b = -4/7 // : -2
b = -4/7/(-2)
b = 2/7
b = 2/7
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