y^2-y=90

Solution for y^2-y=90 equation:

y^2-y=90

We move all terms to the left:

y^2-y-(90)=0

We add all the numbers together, and all the variables

y^2-1y-90=0

a = 1; b = -1; c = -90;
Δ = b2-4ac
Δ = -12-4·1·(-90)
Δ = 361
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$y_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$y_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{361}=19$
$y_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-1)-19}{2*1}=\frac{-18}{2}
=-9
$
$y_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-1)+19}{2*1}=\frac{20}{2}
=10
$