-3x(x+9)=15

Solution for -3x(x+9)=15 equation:

-3x(x+9)=15

We move all terms to the left:

-3x(x+9)-(15)=0

We multiply parentheses

-3x^2-27x-15=0

a = -3; b = -27; c = -15;
Δ = b2-4ac
Δ = -272-4·(-3)·(-15)
Δ = 549
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:

$\sqrt{\Delta}=\sqrt{549}=\sqrt{9*61}=\sqrt{9}*\sqrt{61}=3\sqrt{61}$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-27)-3\sqrt{61}}{2*-3}=\frac{27-3\sqrt{61}}{-6}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-27)+3\sqrt{61}}{2*-3}=\frac{27+3\sqrt{61}}{-6}
$