(3a+7)*(-4a)=0

Solution for (3a+7)*(-4a)=0 equation:

(3a+7)(-4a)=0

We multiply parentheses ..

(-12a^2-28a)=0

We get rid of parentheses

-12a^2-28a=0

a = -12; b = -28; c = 0;
Δ = b2-4ac
Δ = -282-4·(-12)·0
Δ = 784
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$a_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$a_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{784}=28$
$a_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-28)-28}{2*-12}=\frac{0}{-24}
=0
$
$a_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-28)+28}{2*-12}=\frac{56}{-24}
=-2+1/3
$