(x+15)*(x+15)=81

Solution for (x+15)*(x+15)=81 equation:

(x+15)(x+15)=81

We move all terms to the left:

(x+15)(x+15)-(81)=0

We multiply parentheses ..

(+x^2+15x+15x+225)-81=0

We get rid of parentheses

x^2+15x+15x+225-81=0

We add all the numbers together, and all the variables

x^2+30x+144=0

a = 1; b = 30; c = +144;
Δ = b2-4ac
Δ = 302-4·1·144
Δ = 324
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{324}=18$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(30)-18}{2*1}=\frac{-48}{2}
=-24
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(30)+18}{2*1}=\frac{-12}{2}
=-6
$