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15*x-(21*x)-(((5*y)/7)*y) = 0
15*x-21*x+(-5/7)*y^2 = 0
-6*x-5/7*y^2 = 0 // + -5/7*y^2
-6*x = -(-5/7*y^2) // : -6
x = (-(-5/7*y^2))/(-6)
x = -5/42*y^2
x = -5/42*y^2
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