$$u^2-9 = -36$$
Answer
$$ \begin{matrix}u_1 = 3 \sqrt{ 3 } i & u_2 = -3 \sqrt{ 3 } i \\[1 em] \end{matrix} $$
Explanation
$$ \begin{aligned} u^2-9 &= -36&& \text{move all terms to the left hand side } \\[1 em]u^2-9+36 &= 0&& \text{simplify left side} \\[1 em]u^2+27 &= 0&& \\[1 em] \end{aligned} $$
$ x^{2}+27 = 0 $ is a quadratic equation.
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