$$-3x(2x+5) = 21$$
Answer
$$ \begin{matrix}x_1 = -\dfrac{ 5 }{ 4 }+\dfrac{\sqrt{ 31 }}{ 4 }i & x_2 = -\dfrac{ 5 }{ 4 }-\dfrac{\sqrt{ 31 }}{ 4 }i \\[1 em] \end{matrix} $$
Explanation
$$ \begin{aligned} -3x(2x+5) &= 21&& \text{simplify left side} \\[1 em]-(6x^2+15x) &= 21&& \\[1 em]-6x^2-15x &= 21&& \text{move all terms to the left hand side } \\[1 em]-6x^2-15x-21 &= 0&& \\[1 em] \end{aligned} $$
$ -6x^{2}-15x-21 = 0 $ is a quadratic equation.
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