$$3s^3+18s^2+31s+20 = 0$$
Answer
$$ \begin{matrix}s_1 = -3.68853 & s_2 = -1.15574+0.68679i & s_3 = -1.15574-0.68679i \end{matrix} $$
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