(5x+20)(5x-20)=0

Solution for (5x+20)(5x-20)=0 equation:

(5x+20)(5x-20)=0

We use the square of the difference formula

25x^2-400=0

a = 25; b = 0; c = -400;
Δ = b2-4ac
Δ = 02-4·25·(-400)
Δ = 40000
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{40000}=200$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(0)-200}{2*25}=\frac{-200}{50}
=-4
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(0)+200}{2*25}=\frac{200}{50}
=4
$