$$-x^3+3x+20 = 0$$
Answer
$$ \begin{matrix}x_1 = 3.08086 & x_2 = -1.54043+2.02948i & x_3 = -1.54043-2.02948i \end{matrix} $$
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