28=x(28-x)

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

28=x(28-x)

We move all terms to the left:

28-(x(28-x))=0

We add all the numbers together, and all the variables

-(x(-1x+28))+28=0


We calculate terms in parentheses: -(x(-1x+28)), so:

x(-1x+28)

We multiply parentheses

-1x^2+28x

Back to the equation:

-(-1x^2+28x)


We get rid of parentheses

1x^2-28x+28=0

We add all the numbers together, and all the variables

x^2-28x+28=0

a = 1; b = -28; c = +28;
Δ = b2-4ac
Δ = -282-4·1·28
Δ = 672
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:

$\sqrt{\Delta}=\sqrt{672}=\sqrt{16*42}=\sqrt{16}*\sqrt{42}=4\sqrt{42}$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-28)-4\sqrt{42}}{2*1}=\frac{28-4\sqrt{42}}{2}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-28)+4\sqrt{42}}{2*1}=\frac{28+4\sqrt{42}}{2}
$