$$-x^3-12 = 0$$
Answer
$$ \begin{matrix}x_1 = -2.28943 & x_2 = 1.14471+1.9827i & x_3 = 1.14471-1.9827i \end{matrix} $$
Explanation
This polynomial has no rational roots that can be found using Rational Root Test.
Roots were found using qubic formulas.
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Polynomial Equations Solver