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b in (-oo:+oo)
-15 = 2*b-(5*b)-6 // - 2*b-(5*b)-6
5*b-(2*b)-15+6 = 0
5*b-2*b-15+6 = 0
3*b-9 = 0 // + 9
3*b = 9 // : 3
b = 9/3
b = 3
b = 3
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