Given the equation:
$$6 x^{2} + \left(x^{3} + 9 x\right) = 0$$
transform
Take common factor x from the equation
we get:
$$x \left(x^{2} + 6 x + 9\right) = 0$$
then:
$$x_{1} = 0$$
and also
we get the equation
$$x^{2} + 6 x + 9 = 0$$
This equation is of the form
a*x^2 + b*x + c = 0
A quadratic equation can be solved
using the discriminant.
The roots of the quadratic equation:
$$x_{2} = \frac{\sqrt{D} - b}{2 a}$$
$$x_{3} = \frac{- \sqrt{D} - b}{2 a}$$
where D = b^2 - 4*a*c - it is the discriminant.
Because
$$a = 1$$
$$b = 6$$
$$c = 9$$
, then
D = b^2 - 4 * a * c =
(6)^2 - 4 * (1) * (9) = 0
Because D = 0, then the equation has one root.
x = -b/2a = -6/2/(1)
$$x_{2} = -3$$
The final answer for 9*x + x^3 + 6*x^2 = 0:
$$x_{1} = 0$$
$$x_{2} = -3$$