-3x(4x+9)=15

Solution for -3x(4x+9)=15 equation:

-3x(4x+9)=15

We move all terms to the left:

-3x(4x+9)-(15)=0

We multiply parentheses

-12x^2-27x-15=0

a = -12; b = -27; c = -15;
Δ = b2-4ac
Δ = -272-4·(-12)·(-15)
Δ = 9
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{9}=3$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-27)-3}{2*-12}=\frac{24}{-24}
=-1
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-27)+3}{2*-12}=\frac{30}{-24}
=-1+1/4
$