720=(42*21)-(42-2x)(21-2x)

Solution for 720=(42*21)-(42-2x)(21-2x) equation:

720=(42*21)-(42-2x)(21-2x)

We move all terms to the left:

720-((42*21)-(42-2x)(21-2x))=0

We add all the numbers together, and all the variables

-(882-(-2x+42)(-2x+21))+720=0

We multiply parentheses ..

-(882-(+4x^2-42x-84x+882))+720=0


We calculate terms in parentheses: -(882-(+4x^2-42x-84x+882)), so:

882-(+4x^2-42x-84x+882)

determiningTheFunctionDomain
-(+4x^2-42x-84x+882)+882

We get rid of parentheses

-4x^2+42x+84x-882+882

We add all the numbers together, and all the variables

-4x^2+126x

Back to the equation:

-(-4x^2+126x)


We get rid of parentheses

4x^2-126x+720=0

a = 4; b = -126; c = +720;
Δ = b2-4ac
Δ = -1262-4·4·720
Δ = 4356
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{4356}=66$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-126)-66}{2*4}=\frac{60}{8}
=7+1/2
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-126)+66}{2*4}=\frac{192}{8}
=24
$