$$-t^4+56t^2+240 = 0$$
Answer
$$ \begin{matrix}t_1 = 7.74597 & t_2 = -7.74597 & t_3 = 2i \\[1 em] t_4 = -2i & \\[1 em] \end{matrix} $$
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