-x(90-x)=18

Solution for -x(90-x)=18 equation:

-x(90-x)=18

We move all terms to the left:

-x(90-x)-(18)=0

We add all the numbers together, and all the variables

-x(-1x+90)-18=0

We multiply parentheses

1x^2-90x-18=0

We add all the numbers together, and all the variables

x^2-90x-18=0

a = 1; b = -90; c = -18;
Δ = b2-4ac
Δ = -902-4·1·(-18)
Δ = 8172
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:

$\sqrt{\Delta}=\sqrt{8172}=\sqrt{36*227}=\sqrt{36}*\sqrt{227}=6\sqrt{227}$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-90)-6\sqrt{227}}{2*1}=\frac{90-6\sqrt{227}}{2}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-90)+6\sqrt{227}}{2*1}=\frac{90+6\sqrt{227}}{2}
$