$$5x^4-28x^3+56x^2-30 = 0$$
Answer
$$ \begin{matrix}x_1 = -0.62986 & x_2 = 0.93729 & x_3 = 2.64628+1.77776i \\[1 em] x_4 = 2.64628-1.77776i & \\[1 em] \end{matrix} $$
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