28=x^2-18x

Solution for 28=x^2-18x equation:

28=x^2-18x

We move all terms to the left:

28-(x^2-18x)=0

We get rid of parentheses

-x^2+18x+28=0

We add all the numbers together, and all the variables

-1x^2+18x+28=0

a = -1; b = 18; c = +28;
Δ = b2-4ac
Δ = 182-4·(-1)·28
Δ = 436
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:

$\sqrt{\Delta}=\sqrt{436}=\sqrt{4*109}=\sqrt{4}*\sqrt{109}=2\sqrt{109}$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(18)-2\sqrt{109}}{2*-1}=\frac{-18-2\sqrt{109}}{-2}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(18)+2\sqrt{109}}{2*-1}=\frac{-18+2\sqrt{109}}{-2}
$