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

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

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

We move all terms to the left:

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

We add all the numbers together, and all the variables

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

We multiply parentheses ..

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

We get rid of parentheses

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

We add all the numbers together, and all the variables

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