138-3x(X+4)=18

Solution for 138-3x(X+4)=18 equation:

138-3x(x+4)=18

We move all terms to the left:

138-3x(x+4)-(18)=0

We add all the numbers together, and all the variables

-3x(x+4)+120=0

We multiply parentheses

-3x^2-12x+120=0

a = -3; b = -12; c = +120;
Δ = b2-4ac
Δ = -122-4·(-3)·120
Δ = 1584
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:

$\sqrt{\Delta}=\sqrt{1584}=\sqrt{144*11}=\sqrt{144}*\sqrt{11}=12\sqrt{11}$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-12)-12\sqrt{11}}{2*-3}=\frac{12-12\sqrt{11}}{-6}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-12)+12\sqrt{11}}{2*-3}=\frac{12+12\sqrt{11}}{-6}
$