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x in (-oo:+oo)
((3*x)/5)*x = 120 // - 120
((3*x)/5)*x-120 = 0
3/5*x^2 = 120 // : 3/5
x^2 = 200
x^2 = 200 // ^ 1/2
abs(x) = 10*2^(1/2)
x = 10*2^(1/2) or x = -(10*2^(1/2))
x in { 10*2^(1/2), -(10*2^(1/2)) }
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