(y-11)(y-21)=0

Solution for (y-11)(y-21)=0 equation:

(y-11)(y-21)=0

We multiply parentheses ..

(+y^2-21y-11y+231)=0

We get rid of parentheses

y^2-21y-11y+231=0

We add all the numbers together, and all the variables

y^2-32y+231=0

a = 1; b = -32; c = +231;
Δ = b2-4ac
Δ = -322-4·1·231
Δ = 100
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$y_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$y_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{100}=10$
$y_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-32)-10}{2*1}=\frac{22}{2}
=11
$
$y_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-32)+10}{2*1}=\frac{42}{2}
=21
$