(21-x)(20-x)=0

Solution for (21-x)(20-x)=0 equation:

(21-x)(20-x)=0

We add all the numbers together, and all the variables

(-1x+21)(-1x+20)=0

We multiply parentheses ..

(+x^2-20x-21x+420)=0

We get rid of parentheses

x^2-20x-21x+420=0

We add all the numbers together, and all the variables

x^2-41x+420=0

a = 1; b = -41; c = +420;
Δ = b2-4ac
Δ = -412-4·1·420
Δ = 1
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{1}=1$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-41)-1}{2*1}=\frac{40}{2}
=20
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-41)+1}{2*1}=\frac{42}{2}
=21
$