$$r^4+20r^2+64 = 0$$
Answer
$$ \begin{matrix}r_1 = 2i & r_2 = -2i & r_3 = 4i \\[1 em] r_4 = -4i & \\[1 em] \end{matrix} $$
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