(3x+6)(3x+18)=90

Solution for (3x+6)(3x+18)=90 equation:

(3x+6)(3x+18)=90

We move all terms to the left:

(3x+6)(3x+18)-(90)=0

We multiply parentheses ..

(+9x^2+54x+18x+108)-90=0

We get rid of parentheses

9x^2+54x+18x+108-90=0

We add all the numbers together, and all the variables

9x^2+72x+18=0

a = 9; b = 72; c = +18;
Δ = b2-4ac
Δ = 722-4·9·18
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{-(72)-18\sqrt{14}}{2*9}=\frac{-72-18\sqrt{14}}{18}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(72)+18\sqrt{14}}{2*9}=\frac{-72+18\sqrt{14}}{18}
$