(4-y)(5y+9)=0

Solution for (4-y)(5y+9)=0 equation:

(4-y)(5y+9)=0

We add all the numbers together, and all the variables

(-1y+4)(5y+9)=0

We multiply parentheses ..

(-5y^2-9y+20y+36)=0

We get rid of parentheses

-5y^2-9y+20y+36=0

We add all the numbers together, and all the variables

-5y^2+11y+36=0

a = -5; b = 11; c = +36;
Δ = b2-4ac
Δ = 112-4·(-5)·36
Δ = 841
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$y_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$y_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{841}=29$
$y_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(11)-29}{2*-5}=\frac{-40}{-10}
=+4
$
$y_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(11)+29}{2*-5}=\frac{18}{-10}
=-1+4/5
$