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b in (-oo:+oo)
21/4 = 6*b+3*b+3/4 // - 6*b+3*b+3/4
21/4-(6*b)-(3*b)-(3/4) = 0
21/4-6*b-3*b-3/4 = 0
9/2-9*b = 0 // - 9/2
-9*b = -9/2 // : -9
b = -9/2/(-9)
b = 1/2
b = 1/2
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