$$x^4 = -9$$
Answer
$$ \begin{matrix}x_1 = 1.22474+1.22474i & x_2 = 1.22474-1.22474i & x_3 = -1.22474+1.22474i \\[1 em] x_4 = -1.22474-1.22474i & \\[1 em] \end{matrix} $$
Explanation
$$ \begin{aligned} x^4 &= -9&& \text{move all terms to the left hand side } \\[1 em]x^4+9 &= 0&& \\[1 em] \end{aligned} $$
This polynomial has no rational roots that can be found using Rational Root Test.
Roots were found using quartic formulas
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