$$(2x^2-7)(5x^2-3) = 0$$
Answer
$$ \begin{matrix}x_1 = 0.7746 & x_2 = -0.7746 & x_3 = -1.87083 \\[1 em] x_4 = 1.87083 & \\[1 em] \end{matrix} $$
Explanation
$$ \begin{aligned} (2x^2-7)(5x^2-3) &= 0&& \text{simplify left side} \\[1 em]10x^4-6x^2-35x^2+21 &= 0&& \\[1 em]10x^4-41x^2+21 &= 0&& \\[1 em] \end{aligned} $$
This polynomial has no rational roots that can be found using Rational Root Test.
Roots were found using quartic formulas
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Polynomial Equations Solver