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b in (-oo:+oo)
-((5/7)*b)-3 = -81/7 // + -81/7
-((5/7)*b)-(-81/7)-3 = 0
(-5/7)*b+81/7-3 = 0
60/7-5/7*b = 0 // - 60/7
-5/7*b = -60/7 // : -5/7
b = -60/7/(-5/7)
b = 12
b = 12
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