(x+8)(300-20x)=0

Solution for (x+8)(300-20x)=0 equation:

(x+8)(300-20x)=0

We add all the numbers together, and all the variables

(x+8)(-20x+300)=0

We multiply parentheses ..

(-20x^2+300x-160x+2400)=0

We get rid of parentheses

-20x^2+300x-160x+2400=0

We add all the numbers together, and all the variables

-20x^2+140x+2400=0

a = -20; b = 140; c = +2400;
Δ = b2-4ac
Δ = 1402-4·(-20)·2400
Δ = 211600
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{211600}=460$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(140)-460}{2*-20}=\frac{-600}{-40}
=+15
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(140)+460}{2*-20}=\frac{320}{-40}
=-8
$