(x-15)(x+28)=0

Solution for (x-15)(x+28)=0 equation:

(x-15)(x+28)=0

We multiply parentheses ..

(+x^2+28x-15x-420)=0

We get rid of parentheses

x^2+28x-15x-420=0

We add all the numbers together, and all the variables

x^2+13x-420=0

a = 1; b = 13; c = -420;
Δ = b2-4ac
Δ = 132-4·1·(-420)
Δ = 1849
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{1849}=43$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(13)-43}{2*1}=\frac{-56}{2}
=-28
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(13)+43}{2*1}=\frac{30}{2}
=15
$