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5/3-((1/7)*x) = 4/21 // - 4/21
5/3-((1/7)*x)-(4/21) = 0
(-1/7)*x+5/3-4/21 = 0
31/21-1/7*x = 0 // - 31/21
-1/7*x = -31/21 // : -1/7
x = -31/21/(-1/7)
x = 31/3
x = 31/3
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