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8/21 = (10/21)*x+3/7 // - (10/21)*x+3/7
8/21-((10/21)*x)-(3/7) = 0
(-10/21)*x+8/21-3/7 = 0
-10/21*x-1/21 = 0 // + 1/21
-10/21*x = 1/21 // : -10/21
x = 1/21/(-10/21)
x = -1/10
x = -1/10
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