(7-x)(5x+4)=0

Solution for (7-x)(5x+4)=0 equation:

(7-x)(5x+4)=0

We add all the numbers together, and all the variables

(-1x+7)(5x+4)=0

We multiply parentheses ..

(-5x^2-4x+35x+28)=0

We get rid of parentheses

-5x^2-4x+35x+28=0

We add all the numbers together, and all the variables

-5x^2+31x+28=0

a = -5; b = 31; c = +28;
Δ = b2-4ac
Δ = 312-4·(-5)·28
Δ = 1521
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{1521}=39$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(31)-39}{2*-5}=\frac{-70}{-10}
=+7
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(31)+39}{2*-5}=\frac{8}{-10}
=-4/5
$