$$x^4+3x^2-18 = 0$$
Answer
$$ \begin{matrix}x_1 = 1.73205 & x_2 = -1.73205 & x_3 = 2.44949i \\[1 em] x_4 = -2.44949i & \\[1 em] \end{matrix} $$
Explanation
This polynomial has no rational roots that can be found using Rational Root Test.
Roots were found using quartic formulas
This page was created using
Polynomial Equations Solver