$$(s+2)(s^3+2s^2+s-2) = 0$$
Answer
$$ \begin{matrix}s_1 = -2 & s_2 = 0.69562 & s_3 = -1.34781+1.02885i \\[1 em] s_4 = -1.34781-1.02885i & \\[1 em] \end{matrix} $$
Explanation
$$ \begin{aligned} (s+2)(s^3+2s^2+s-2) &= 0&& \text{simplify left side} \\[1 em]s^4+4s^3+5s^2-4 &= 0&& \\[1 em] \end{aligned} $$
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