28=(x+5)(x-7)

Solution for 28=(x+5)(x-7) equation:

28=(x+5)(x-7)

We move all terms to the left:

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

We multiply parentheses ..

-((+x^2-7x+5x-35))+28=0


We calculate terms in parentheses: -((+x^2-7x+5x-35)), so:

(+x^2-7x+5x-35)

We get rid of parentheses

x^2-7x+5x-35

We add all the numbers together, and all the variables

x^2-2x-35

Back to the equation:

-(x^2-2x-35)


We get rid of parentheses

-x^2+2x+35+28=0

We add all the numbers together, and all the variables

-1x^2+2x+63=0

a = -1; b = 2; c = +63;
Δ = b2-4ac
Δ = 22-4·(-1)·63
Δ = 256
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{256}=16$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(2)-16}{2*-1}=\frac{-18}{-2}
=+9
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(2)+16}{2*-1}=\frac{14}{-2}
=-7
$