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(1/5)*x-(2*x)-((3/7)*y)+(6/7)*y = 0
(1/5)*x-2*x+(-3/7)*y+(6/7)*y = 0
3/7*y-9/5*x = 0 // - 3/7*y
-9/5*x = -(3/7*y) // : -9/5
x = (-(3/7*y))/(-9/5)
x = 5/21*y
x = 5/21*y
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