$$4x^3+8x^2+20x+8 = 0$$
Answer
$$ \begin{matrix}x_1 = -0.46682 & x_2 = -0.76659+1.92266i & x_3 = -0.76659-1.92266i \end{matrix} $$
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