(3x+12)(3x)=90

Solution for (3x+12)(3x)=90 equation:

(3x+12)(3x)=90

We move all terms to the left:

(3x+12)(3x)-(90)=0

We multiply parentheses

9x^2+36x-90=0

a = 9; b = 36; c = -90;
Δ = b2-4ac
Δ = 362-4·9·(-90)
Δ = 4536
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

The end solution:

$\sqrt{\Delta}=\sqrt{4536}=\sqrt{324*14}=\sqrt{324}*\sqrt{14}=18\sqrt{14}$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(36)-18\sqrt{14}}{2*9}=\frac{-36-18\sqrt{14}}{18}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(36)+18\sqrt{14}}{2*9}=\frac{-36+18\sqrt{14}}{18}
$