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b in (-oo:+oo)
(2/7)*b-((1/3)*b)+8 = 502/63 // - 502/63
(2/7)*b-((1/3)*b)-(502/63)+8 = 0
(-1/3)*b+(2/7)*b-502/63+8 = 0
2/63-1/21*b = 0 // - 2/63
-1/21*b = -2/63 // : -1/21
b = -2/63/(-1/21)
b = 2/3
b = 2/3
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