$$s^4-35s^2+24 = 0$$
Answer
$$ \begin{matrix}s_1 = -0.83648 & s_2 = 0.83648 & s_3 = 5.85665 \\[1 em] s_4 = -5.85665 & \\[1 em] \end{matrix} $$
Explanation
This polynomial has no rational roots that can be found using Rational Root Test.
Roots were found using quartic formulas
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Polynomial Equations Solver