n^2-9n=90

Solution for n^2-9n=90 equation:

n^2-9n=90

We move all terms to the left:

n^2-9n-(90)=0

a = 1; b = -9; c = -90;
Δ = b2-4ac
Δ = -92-4·1·(-90)
Δ = 441
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$n_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$n_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{441}=21$
$n_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-9)-21}{2*1}=\frac{-12}{2}
=-6
$
$n_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-9)+21}{2*1}=\frac{30}{2}
=15
$