(-3x-15)(x+7)=0

Solution for (-3x-15)(x+7)=0 equation:

(-3x-15)(x+7)=0

We multiply parentheses ..

(-3x^2-21x-15x-105)=0

We get rid of parentheses

-3x^2-21x-15x-105=0

We add all the numbers together, and all the variables

-3x^2-36x-105=0

a = -3; b = -36; c = -105;
Δ = b2-4ac
Δ = -362-4·(-3)·(-105)
Δ = 36
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{36}=6$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-36)-6}{2*-3}=\frac{30}{-6}
=-5
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-36)+6}{2*-3}=\frac{42}{-6}
=-7
$