-x(5x+6)=-18

Solution for -x(5x+6)=-18 equation:

-x(5x+6)=-18

We move all terms to the left:

-x(5x+6)-(-18)=0

We add all the numbers together, and all the variables

-x(5x+6)+18=0

We multiply parentheses

-5x^2-6x+18=0

a = -5; b = -6; c = +18;
Δ = b2-4ac
Δ = -62-4·(-5)·18
Δ = 396
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:

$\sqrt{\Delta}=\sqrt{396}=\sqrt{36*11}=\sqrt{36}*\sqrt{11}=6\sqrt{11}$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-6)-6\sqrt{11}}{2*-5}=\frac{6-6\sqrt{11}}{-10}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-6)+6\sqrt{11}}{2*-5}=\frac{6+6\sqrt{11}}{-10}
$