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x in (-oo:+oo)
(x^4)/3+(3*x^4)/3-28 = 0
4/3*x^4 = 28 // : 4/3
x^4 = 21
x^4 = 21 // ^ 1/4
abs(x) = 21^(1/4)
x = 21^(1/4) or x = -21^(1/4)
x in { 21^(1/4), -21^(1/4) }
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