-3x(x+8)-x=12

Solution for -3x(x+8)-x=12 equation:

-3x(x+8)-x=12

We move all terms to the left:

-3x(x+8)-x-(12)=0

We add all the numbers together, and all the variables

-1x-3x(x+8)-12=0

We multiply parentheses

-3x^2-1x-24x-12=0

We add all the numbers together, and all the variables

-3x^2-25x-12=0

a = -3; b = -25; c = -12;
Δ = b2-4ac
Δ = -252-4·(-3)·(-12)
Δ = 481
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-25)-\sqrt{481}}{2*-3}=\frac{25-\sqrt{481}}{-6}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-25)+\sqrt{481}}{2*-3}=\frac{25+\sqrt{481}}{-6}
$