396=(x+3)(x+7)

Solution for 396=(x+3)(x+7) equation:

396=(x+3)(x+7)

We move all terms to the left:

396-((x+3)(x+7))=0

We multiply parentheses ..

-((+x^2+7x+3x+21))+396=0


We calculate terms in parentheses: -((+x^2+7x+3x+21)), so:

(+x^2+7x+3x+21)

We get rid of parentheses

x^2+7x+3x+21

We add all the numbers together, and all the variables

x^2+10x+21

Back to the equation:

-(x^2+10x+21)


We get rid of parentheses

-x^2-10x-21+396=0

We add all the numbers together, and all the variables

-1x^2-10x+375=0

a = -1; b = -10; c = +375;
Δ = b2-4ac
Δ = -102-4·(-1)·375
Δ = 1600
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{1600}=40$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-10)-40}{2*-1}=\frac{-30}{-2}
=+15
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-10)+40}{2*-1}=\frac{50}{-2}
=-25
$