(x+3)(x-6)=360

Solution for (x+3)(x-6)=360 equation:

(x+3)(x-6)=360

We move all terms to the left:

(x+3)(x-6)-(360)=0

We multiply parentheses ..

(+x^2-6x+3x-18)-360=0

We get rid of parentheses

x^2-6x+3x-18-360=0

We add all the numbers together, and all the variables

x^2-3x-378=0

a = 1; b = -3; c = -378;
Δ = b2-4ac
Δ = -32-4·1·(-378)
Δ = 1521
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{1521}=39$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-3)-39}{2*1}=\frac{-36}{2}
=-18
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-3)+39}{2*1}=\frac{42}{2}
=21
$