(3x+15)(3x+15)=90

Solution for (3x+15)(3x+15)=90 equation:

(3x+15)(3x+15)=90

We move all terms to the left:

(3x+15)(3x+15)-(90)=0

We multiply parentheses ..

(+9x^2+45x+45x+225)-90=0

We get rid of parentheses

9x^2+45x+45x+225-90=0

We add all the numbers together, and all the variables

9x^2+90x+135=0

a = 9; b = 90; c = +135;
Δ = b2-4ac
Δ = 902-4·9·135
Δ = 3240
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:

$\sqrt{\Delta}=\sqrt{3240}=\sqrt{324*10}=\sqrt{324}*\sqrt{10}=18\sqrt{10}$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(90)-18\sqrt{10}}{2*9}=\frac{-90-18\sqrt{10}}{18}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(90)+18\sqrt{10}}{2*9}=\frac{-90+18\sqrt{10}}{18}
$