X(x-4)=28

Solution for X(x-4)=28 equation:

X(X-4)=28

We move all terms to the left:

X(X-4)-(28)=0

We multiply parentheses

X^2-4X-28=0

a = 1; b = -4; c = -28;
Δ = b2-4ac
Δ = -42-4·1·(-28)
Δ = 128
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:

$\sqrt{\Delta}=\sqrt{128}=\sqrt{64*2}=\sqrt{64}*\sqrt{2}=8\sqrt{2}$
$X_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-4)-8\sqrt{2}}{2*1}=\frac{4-8\sqrt{2}}{2}
$
$X_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-4)+8\sqrt{2}}{2*1}=\frac{4+8\sqrt{2}}{2}
$