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(5/6)*x-((2/3)*x)+6/7 = 19/21 // - 19/21
(5/6)*x-((2/3)*x)+6/7-(19/21) = 0
(5/6)*x+(-2/3)*x+6/7-19/21 = 0
1/6*x-1/21 = 0 // + 1/21
1/6*x = 1/21 // : 1/6
x = 1/21/1/6
x = 2/7
x = 2/7
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