-3x(x+15)=15

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

-3x(x+15)=15

We move all terms to the left:

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

We multiply parentheses

-3x^2-45x-15=0

a = -3; b = -45; c = -15;
Δ = b2-4ac
Δ = -452-4·(-3)·(-15)
Δ = 1845
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:

$\sqrt{\Delta}=\sqrt{1845}=\sqrt{9*205}=\sqrt{9}*\sqrt{205}=3\sqrt{205}$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-45)-3\sqrt{205}}{2*-3}=\frac{45-3\sqrt{205}}{-6}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-45)+3\sqrt{205}}{2*-3}=\frac{45+3\sqrt{205}}{-6}
$