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

Solution for (X-21)(x+1)=0 equation:

(X-21)(X+1)=0

We multiply parentheses ..

(+X^2+X-21X-21)=0

We get rid of parentheses

X^2+X-21X-21=0

We add all the numbers together, and all the variables

X^2-20X-21=0

a = 1; b = -20; c = -21;
Δ = b2-4ac
Δ = -202-4·1·(-21)
Δ = 484
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$X_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$X_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{484}=22$
$X_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-20)-22}{2*1}=\frac{-2}{2}
=-1
$
$X_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-20)+22}{2*1}=\frac{42}{2}
=21
$