7(x+28)(x-21)=0

Solution for 7(x+28)(x-21)=0 equation:

7(x+28)(x-21)=0

We multiply parentheses ..

7(+x^2-21x+28x-588)=0

We multiply parentheses

7x^2-147x+196x-4116=0

We add all the numbers together, and all the variables

7x^2+49x-4116=0

a = 7; b = 49; c = -4116;
Δ = b2-4ac
Δ = 492-4·7·(-4116)
Δ = 117649
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{117649}=343$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(49)-343}{2*7}=\frac{-392}{14}
=-28
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(49)+343}{2*7}=\frac{294}{14}
=21
$