-x2-20x-91=0

Solution for -x2-20x-91=0 equation:

-x2-20x-91=0

We add all the numbers together, and all the variables

-1x^2-20x-91=0

a = -1; b = -20; c = -91;
Δ = b2-4ac
Δ = -202-4·(-1)·(-91)
Δ = 36
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{36}=6$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-20)-6}{2*-1}=\frac{14}{-2}
=-7
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-20)+6}{2*-1}=\frac{26}{-2}
=-13
$