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3*x+18/y = 21 // - 21
3*x+18/y-21 = 0
3*x+18*y^-1-21 = 0 // - 18*y^-1-21
3*x = -(18*y^-1-21) // : 3
x = (-(18*y^-1-21))/3
x = (21-(18*y^-1))/3
x = (21-(18*y^-1))/3
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