(x+5)(7(x)-21)=0

Solution for (x+5)(7(x)-21)=0 equation:

(x+5)(7(x)-21)=0

We multiply parentheses ..

(+7x^2-21x+35x-105)=0

We get rid of parentheses

7x^2-21x+35x-105=0

We add all the numbers together, and all the variables

7x^2+14x-105=0

a = 7; b = 14; c = -105;
Δ = b2-4ac
Δ = 142-4·7·(-105)
Δ = 3136
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{3136}=56$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(14)-56}{2*7}=\frac{-70}{14}
=-5
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(14)+56}{2*7}=\frac{42}{14}
=3
$