3(5x^2-28x-12)=0

Solution for 3(5x^2-28x-12)=0 equation:

3(5x^2-28x-12)=0

We multiply parentheses

15x^2-84x-36=0

a = 15; b = -84; c = -36;
Δ = b2-4ac
Δ = -842-4·15·(-36)
Δ = 9216
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{9216}=96$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-84)-96}{2*15}=\frac{-12}{30}
=-2/5
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-84)+96}{2*15}=\frac{180}{30}
=6
$