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

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

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

We add all the numbers together, and all the variables

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

We multiply parentheses ..

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

We get rid of parentheses

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

We add all the numbers together, and all the variables

-5y^2+31y+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:
$y_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$y_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

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