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b in (-oo:+oo)
-b-10 = 2*b-31 // - 2*b-31
31-b-(2*b)-10 = 0
31-b-2*b-10 = 0
21-3*b = 0 // - 21
-3*b = -21 // : -3
b = -21/(-3)
b = 7
b = 7
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