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b in (-oo:+oo)
-2*b = -8/5 // + -8/5
-2*b-(-8/5) = 0
8/5-2*b = 0
8/5-2*b = 0 // - 8/5
-2*b = -8/5 // : -2
b = -8/5/(-2)
b = 4/5
b = 4/5
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