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d in (-oo:+oo)
1/8-((5/6)*d) = 121/168 // - 121/168
1/8-((5/6)*d)-(121/168) = 0
(-5/6)*d+1/8-121/168 = 0
-5/6*d-25/42 = 0 // + 25/42
-5/6*d = 25/42 // : -5/6
d = 25/42/(-5/6)
d = -5/7
d = -5/7
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