(18+3x)*x=120

Solution for (18+3x)*x=120 equation:

(18+3x)*x=120

We move all terms to the left:

(18+3x)*x-(120)=0

We add all the numbers together, and all the variables

(3x+18)*x-120=0

We multiply parentheses

3x^2+18x-120=0

a = 3; b = 18; c = -120;
Δ = b2-4ac
Δ = 182-4·3·(-120)
Δ = 1764
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{1764}=42$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(18)-42}{2*3}=\frac{-60}{6}
=-10
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(18)+42}{2*3}=\frac{24}{6}
=4
$