(x-10)(3x+15)=300

Solution for (x-10)(3x+15)=300 equation:

(x-10)(3x+15)=300

We move all terms to the left:

(x-10)(3x+15)-(300)=0

We multiply parentheses ..

(+3x^2+15x-30x-150)-300=0

We get rid of parentheses

3x^2+15x-30x-150-300=0

We add all the numbers together, and all the variables

3x^2-15x-450=0

a = 3; b = -15; c = -450;
Δ = b2-4ac
Δ = -152-4·3·(-450)
Δ = 5625
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{5625}=75$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-15)-75}{2*3}=\frac{-60}{6}
=-10
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-15)+75}{2*3}=\frac{90}{6}
=15
$