-3x(2x+5)=-15

Solution for -3x(2x+5)=-15 equation:

-3x(2x+5)=-15

We move all terms to the left:

-3x(2x+5)-(-15)=0

We add all the numbers together, and all the variables

-3x(2x+5)+15=0

We multiply parentheses

-6x^2-15x+15=0

a = -6; b = -15; c = +15;
Δ = b2-4ac
Δ = -152-4·(-6)·15
Δ = 585
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:

$\sqrt{\Delta}=\sqrt{585}=\sqrt{9*65}=\sqrt{9}*\sqrt{65}=3\sqrt{65}$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-15)-3\sqrt{65}}{2*-6}=\frac{15-3\sqrt{65}}{-12}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-15)+3\sqrt{65}}{2*-6}=\frac{15+3\sqrt{65}}{-12}
$