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x in (-oo:+oo)
((3*x)/5)*x = 15 // - 15
((3*x)/5)*x-15 = 0
3/5*x^2 = 15 // : 3/5
x^2 = 25
x^2 = 25 // ^ 1/2
abs(x) = 5
x = 5 or x = -5
x in { 5, -5 }
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