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(2/3)*x-((5/7)*x)+4 = 677/168 // - 677/168
(2/3)*x-((5/7)*x)-(677/168)+4 = 0
(-5/7)*x+(2/3)*x-677/168+4 = 0
-1/21*x-5/168 = 0 // + 5/168
-1/21*x = 5/168 // : -1/21
x = 5/168/(-1/21)
x = -5/8
x = -5/8
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