90=x^2-17x

Solution for 90=x^2-17x equation:

90=x^2-17x

We move all terms to the left:

90-(x^2-17x)=0

We get rid of parentheses

-x^2+17x+90=0

We add all the numbers together, and all the variables

-1x^2+17x+90=0

a = -1; b = 17; c = +90;
Δ = b2-4ac
Δ = 172-4·(-1)·90
Δ = 649
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(17)-\sqrt{649}}{2*-1}=\frac{-17-\sqrt{649}}{-2}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(17)+\sqrt{649}}{2*-1}=\frac{-17+\sqrt{649}}{-2}
$