-99=90m-9m^2

Solution for -99=90m-9m^2 equation:

-99=90m-9m^2

We move all terms to the left:

-99-(90m-9m^2)=0

We get rid of parentheses

9m^2-90m-99=0

a = 9; b = -90; c = -99;
Δ = b2-4ac
Δ = -902-4·9·(-99)
Δ = 11664
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$m_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$m_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{11664}=108$
$m_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-90)-108}{2*9}=\frac{-18}{18}
=-1
$
$m_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-90)+108}{2*9}=\frac{198}{18}
=11
$