72=9x^2

Solution for 72=9x^2 equation:

72=9x^2

We move all terms to the left:

72-(9x^2)=0

a = -9; b = 0; c = +72;
Δ = b2-4ac
Δ = 02-4·(-9)·72
Δ = 2592
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

The end solution:

$\sqrt{\Delta}=\sqrt{2592}=\sqrt{1296*2}=\sqrt{1296}*\sqrt{2}=36\sqrt{2}$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(0)-36\sqrt{2}}{2*-9}=\frac{0-36\sqrt{2}}{-18}
=-\frac{36\sqrt{2}}{-18}
=-\frac{2\sqrt{2}}{-1}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(0)+36\sqrt{2}}{2*-9}=\frac{0+36\sqrt{2}}{-18}
=\frac{36\sqrt{2}}{-18}
=\frac{2\sqrt{2}}{-1}
$