9x²+36x-108>0 inequation

Given the inequality:
$$\left(9 x^{2} + 36 x\right) - 108 > 0$$
To solve this inequality, we must first solve the corresponding equation:
$$\left(9 x^{2} + 36 x\right) - 108 = 0$$
Solve:
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:
$$x_{1} = \frac{\sqrt{D} - b}{2 a}$$
$$x_{2} = \frac{- \sqrt{D} - b}{2 a}$$
where D = b^2 - 4*a*c - it is the discriminant.
Because
$$a = 9$$
$$b = 36$$
$$c = -108$$
, then

D = b^2 - 4 * a * c = 

(36)^2 - 4 * (9) * (-108) = 5184

Because D > 0, then the equation has two roots.

x1 = (-b + sqrt(D)) / (2*a)

x2 = (-b - sqrt(D)) / (2*a)

or
$$x_{1} = 2$$
$$x_{2} = -6$$
$$x_{1} = 2$$
$$x_{2} = -6$$
$$x_{1} = 2$$
$$x_{2} = -6$$
This roots
$$x_{2} = -6$$
$$x_{1} = 2$$
is the points with change the sign of the inequality expression.
First define with the sign to the leftmost point:
$$x_{0} < x_{2}$$
For example, let's take the point
$$x_{0} = x_{2} - \frac{1}{10}$$
=
$$-6 + - \frac{1}{10}$$
=
$$- \frac{61}{10}$$
substitute to the expression
$$\left(9 x^{2} + 36 x\right) - 108 > 0$$
$$-108 + \left(\frac{\left(-61\right) 36}{10} + 9 \left(- \frac{61}{10}\right)^{2}\right) > 0$$

729    
--- > 0
100    

one of the solutions of our inequality is:
$$x < -6$$

 _____           _____          
      \         /
-------ο-------ο-------
       x2      x1

Other solutions will get with the changeover to the next point
etc.
The answer:
$$x < -6$$
$$x > 2$$