9x^2-72=18x

Solution for 9x^2-72=18x equation:

9x^2-72=18x

We move all terms to the left:

9x^2-72-(18x)=0

a = 9; b = -18; c = -72;
Δ = b2-4ac
Δ = -182-4·9·(-72)
Δ = 2916
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{2916}=54$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-18)-54}{2*9}=\frac{-36}{18}
=-2
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-18)+54}{2*9}=\frac{72}{18}
=4
$