(3x-9)(5x+25)=0

Solution for (3x-9)(5x+25)=0 equation:

(3x-9)(5x+25)=0

We multiply parentheses ..

(+15x^2+75x-45x-225)=0

We get rid of parentheses

15x^2+75x-45x-225=0

We add all the numbers together, and all the variables

15x^2+30x-225=0

a = 15; b = 30; c = -225;
Δ = b2-4ac
Δ = 302-4·15·(-225)
Δ = 14400
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{14400}=120$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(30)-120}{2*15}=\frac{-150}{30}
=-5
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(30)+120}{2*15}=\frac{90}{30}
=3
$