(3x-15)(4x+60)=0

Solution for (3x-15)(4x+60)=0 equation:

(3x-15)(4x+60)=0

We multiply parentheses ..

(+12x^2+180x-60x-900)=0

We get rid of parentheses

12x^2+180x-60x-900=0

We add all the numbers together, and all the variables

12x^2+120x-900=0

a = 12; b = 120; c = -900;
Δ = b2-4ac
Δ = 1202-4·12·(-900)
Δ = 57600
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{57600}=240$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(120)-240}{2*12}=\frac{-360}{24}
=-15
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(120)+240}{2*12}=\frac{120}{24}
=5
$