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

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

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

We add all the numbers together, and all the variables

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

We multiply parentheses ..

(-4x^2+15x-16x+60)=0

We get rid of parentheses

-4x^2+15x-16x+60=0

We add all the numbers together, and all the variables

-4x^2-1x+60=0

a = -4; b = -1; c = +60;
Δ = b2-4ac
Δ = -12-4·(-4)·60
Δ = 961
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{961}=31$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-1)-31}{2*-4}=\frac{-30}{-8}
=3+3/4
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-1)+31}{2*-4}=\frac{32}{-8}
=-4
$