x(x-12)=20

Solution for x(x-12)=20 equation:

x(x-12)=20

We move all terms to the left:

x(x-12)-(20)=0

We multiply parentheses

x^2-12x-20=0

a = 1; b = -12; c = -20;
Δ = b2-4ac
Δ = -122-4·1·(-20)
Δ = 224
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

The end solution:

$\sqrt{\Delta}=\sqrt{224}=\sqrt{16*14}=\sqrt{16}*\sqrt{14}=4\sqrt{14}$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-12)-4\sqrt{14}}{2*1}=\frac{12-4\sqrt{14}}{2}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-12)+4\sqrt{14}}{2*1}=\frac{12+4\sqrt{14}}{2}
$