(x-21)*(x-3)=0

Solution for (x-21)*(x-3)=0 equation:

(x-21)(x-3)=0

We multiply parentheses ..

(+x^2-3x-21x+63)=0

We get rid of parentheses

x^2-3x-21x+63=0

We add all the numbers together, and all the variables

x^2-24x+63=0

a = 1; b = -24; c = +63;
Δ = b2-4ac
Δ = -242-4·1·63
Δ = 324
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{324}=18$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-24)-18}{2*1}=\frac{6}{2}
=3
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-24)+18}{2*1}=\frac{42}{2}
=21
$