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x in (-oo:+oo)
(x/3)*x = 18 // - 18
(x/3)*x-18 = 0
1/3*x^2 = 18 // : 1/3
x^2 = 54
x^2 = 54 // ^ 1/2
abs(x) = 3*6^(1/2)
x = 3*6^(1/2) or x = -(3*6^(1/2))
x in { 3*6^(1/2), -(3*6^(1/2)) }
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