13=x^2-8x

Solution for 13=x^2-8x equation:

13=x^2-8x

We move all terms to the left:

13-(x^2-8x)=0

We get rid of parentheses

-x^2+8x+13=0

We add all the numbers together, and all the variables

-1x^2+8x+13=0

a = -1; b = 8; c = +13;
Δ = b2-4ac
Δ = 82-4·(-1)·13
Δ = 116
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:

$\sqrt{\Delta}=\sqrt{116}=\sqrt{4*29}=\sqrt{4}*\sqrt{29}=2\sqrt{29}$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(8)-2\sqrt{29}}{2*-1}=\frac{-8-2\sqrt{29}}{-2}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(8)+2\sqrt{29}}{2*-1}=\frac{-8+2\sqrt{29}}{-2}
$