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b in (-oo:+oo)
9-(7*b) = -2*b-21 // + -2*b-21
9-(7*b)-(-2*b)+21 = 0
2*b-7*b+9+21 = 0
30-5*b = 0 // - 30
-5*b = -30 // : -5
b = -30/(-5)
b = 6
b = 6
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