(a+10)(a-20)=0

Solution for (a+10)(a-20)=0 equation:

(a+10)(a-20)=0

We multiply parentheses ..

(+a^2-20a+10a-200)=0

We get rid of parentheses

a^2-20a+10a-200=0

We add all the numbers together, and all the variables

a^2-10a-200=0

a = 1; b = -10; c = -200;
Δ = b2-4ac
Δ = -102-4·1·(-200)
Δ = 900
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$a_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$a_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{900}=30$
$a_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-10)-30}{2*1}=\frac{-20}{2}
=-10
$
$a_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-10)+30}{2*1}=\frac{40}{2}
=20
$