$$x^4-x = 12$$
Answer
$$ \begin{matrix}x_1 = 1.93198 & x_2 = -1.78765 & x_3 = -0.07217+1.86261i \\[1 em] x_4 = -0.07217-1.86261i & \\[1 em] \end{matrix} $$
Explanation
$$ \begin{aligned} x^4-x &= 12&& \text{move all terms to the left hand side } \\[1 em]x^4-x-12 &= 0&& \\[1 em] \end{aligned} $$
This polynomial has no rational roots that can be found using Rational Root Test.
Roots were found using quartic formulas
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Polynomial Equations Solver