$$2n^3+4+5n^3+2 = 0$$
Answer
$$ \begin{matrix}n_1 = -0.94991 & n_2 = 0.47496+0.82265i & n_3 = 0.47496-0.82265i \end{matrix} $$
Explanation
$$ \begin{aligned} 2n^3+4+5n^3+2 &= 0&& \text{simplify left side} \\[1 em]7n^3+6 &= 0&& \\[1 em] \end{aligned} $$
This polynomial has no rational roots that can be found using Rational Root Test.
Roots were found using qubic formulas.
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Polynomial Equations Solver