81=100x^2

Solution for 81=100x^2 equation:

81=100x^2

We move all terms to the left:

81-(100x^2)=0

a = -100; b = 0; c = +81;
Δ = b2-4ac
Δ = 02-4·(-100)·81
Δ = 32400
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{32400}=180$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(0)-180}{2*-100}=\frac{-180}{-200}
=9/10
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(0)+180}{2*-100}=\frac{180}{-200}
=-9/10
$