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b in (-oo:+oo)
(2/7)*b-((3/7)*b)+3/8 = 253/504 // - 253/504
(2/7)*b-((3/7)*b)+3/8-(253/504) = 0
(2/7)*b+(-3/7)*b+3/8-253/504 = 0
-1/7*b-8/63 = 0 // + 8/63
-1/7*b = 8/63 // : -1/7
b = 8/63/(-1/7)
b = -8/9
b = -8/9
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