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x in (-oo:+oo)
(-1/7)*x+7/8 = 139/168 // - 139/168
(-1/7)*x+7/8-(139/168) = 0
(-1/7)*x+7/8-139/168 = 0
1/21-1/7*x = 0 // - 1/21
-1/7*x = -1/21 // : -1/7
x = -1/21/(-1/7)
x = 1/3
x = 1/3
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