(3a-12)(a+15)=0

Solution for (3a-12)(a+15)=0 equation:

(3a-12)(a+15)=0

We multiply parentheses ..

(+3a^2+45a-12a-180)=0

We get rid of parentheses

3a^2+45a-12a-180=0

We add all the numbers together, and all the variables

3a^2+33a-180=0

a = 3; b = 33; c = -180;
Δ = b2-4ac
Δ = 332-4·3·(-180)
Δ = 3249
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$a_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$a_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{3249}=57$
$a_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(33)-57}{2*3}=\frac{-90}{6}
=-15
$
$a_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(33)+57}{2*3}=\frac{24}{6}
=4
$