-x^2=81x

Solution for -x^2=81x equation:

-x^2=81x

We move all terms to the left:

-x^2-(81x)=0

We add all the numbers together, and all the variables

-1x^2-81x=0

a = -1; b = -81; c = 0;
Δ = b2-4ac
Δ = -812-4·(-1)·0
Δ = 6561
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{6561}=81$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-81)-81}{2*-1}=\frac{0}{-2}
=0
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-81)+81}{2*-1}=\frac{162}{-2}
=-81
$