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b in (-oo:+oo)
13-(b/6) = (20.5*b)/4 // - (20.5*b)/4
13-(b/6)-((20.5*b)/4) = 0
13-(b/6)-5.125*b = 0
13-5.29166667*b = 0 // - 13
-5.29166667*b = -13 // : -5.29166667
b = -13/(-5.29166667)
b = 2.45669291
b = 2.45669291
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