$$x^4-2^3-39^2+8x+140 = 0$$
Answer
$$ \begin{matrix}x_1 = 6.05096 & x_2 = -6.15829 & x_3 = 0.05366+6.10509i \\[1 em] x_4 = 0.05366-6.10509i & \\[1 em] \end{matrix} $$
Explanation
$$ \begin{aligned} x^4-2^3-39^2+8x+140 &= 0&& \text{simplify left side} \\[1 em]x^4-8-1521+8x+140 &= 0&& \\[1 em]x^4+8x-1389 &= 0&& \\[1 em] \end{aligned} $$
This polynomial has no rational roots that can be found using Rational Root Test.
Roots were found using quartic formulas
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