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b in (-oo:+oo)
(1/3)*b-4/7 = -221/252 // + -221/252
(1/3)*b-4/7-(-221/252) = 0
(1/3)*b-4/7+221/252 = 0
1/3*b+11/36 = 0 // - 11/36
1/3*b = -11/36 // : 1/3
b = -11/36/1/3
b = -11/12
b = -11/12
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