$$5u^2 = -20$$
Answer
$$ \begin{matrix}u_1 = 2 i & u_2 = -2 i \\[1 em] \end{matrix} $$
Explanation
$$ \begin{aligned} 5u^2 &= -20&& \text{move all terms to the left hand side } \\[1 em]5u^2+20 &= 0&& \\[1 em] \end{aligned} $$
$ 5x^{2}+20 = 0 $ is a quadratic equation.
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