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b in (-oo:+oo)
9-(6*b) = 6*b+129 // - 6*b+129
9-(6*b)-(6*b)-129 = 0
9-6*b-6*b-129 = 0
-12*b-120 = 0 // + 120
-12*b = 120 // : -12
b = 120/(-12)
b = -10
b = -10
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