90=x^2-13x

Solution for 90=x^2-13x equation:

90=x^2-13x

We move all terms to the left:

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

We get rid of parentheses

-x^2+13x+90=0

We add all the numbers together, and all the variables

-1x^2+13x+90=0

a = -1; b = 13; c = +90;
Δ = b2-4ac
Δ = 132-4·(-1)·90
Δ = 529
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{529}=23$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(13)-23}{2*-1}=\frac{-36}{-2}
=+18
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(13)+23}{2*-1}=\frac{10}{-2}
=-5
$