9a2-18a-91=0

Solution for 9a2-18a-91=0 equation:

9a^2-18a-91=0

a = 9; b = -18; c = -91;
Δ = b2-4ac
Δ = -182-4·9·(-91)
Δ = 3600
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$a_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$a_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{3600}=60$
$a_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-18)-60}{2*9}=\frac{-42}{18}
=-2+1/3
$
$a_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-18)+60}{2*9}=\frac{78}{18}
=4+1/3
$