X(x-9)=18

Solution for X(x-9)=18 equation:

X(X-9)=18

We move all terms to the left:

X(X-9)-(18)=0

We multiply parentheses

X^2-9X-18=0

a = 1; b = -9; c = -18;
Δ = b2-4ac
Δ = -92-4·1·(-18)
Δ = 153
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:

$\sqrt{\Delta}=\sqrt{153}=\sqrt{9*17}=\sqrt{9}*\sqrt{17}=3\sqrt{17}$
$X_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-9)-3\sqrt{17}}{2*1}=\frac{9-3\sqrt{17}}{2}
$
$X_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-9)+3\sqrt{17}}{2*1}=\frac{9+3\sqrt{17}}{2}
$