(x+4)(15-3x)=0

Solution for (x+4)(15-3x)=0 equation:

(x+4)(15-3x)=0

We add all the numbers together, and all the variables

(x+4)(-3x+15)=0

We multiply parentheses ..

(-3x^2+15x-12x+60)=0

We get rid of parentheses

-3x^2+15x-12x+60=0

We add all the numbers together, and all the variables

-3x^2+3x+60=0

a = -3; b = 3; c = +60;
Δ = b2-4ac
Δ = 32-4·(-3)·60
Δ = 729
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{729}=27$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(3)-27}{2*-3}=\frac{-30}{-6}
=+5
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(3)+27}{2*-3}=\frac{24}{-6}
=-4
$