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9-3*x-((5/7)*x) = 137/21 // - 137/21
9-3*x-((5/7)*x)-(137/21) = 0
(-5/7)*x-3*x-137/21+9 = 0
52/21-26/7*x = 0 // - 52/21
-26/7*x = -52/21 // : -26/7
x = -52/21/(-26/7)
x = 2/3
x = 2/3
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