(x+1)(x+3)=12

Solution for (x+1)(x+3)=12 equation:

(x+1)(x+3)=12

We move all terms to the left:

(x+1)(x+3)-(12)=0

We multiply parentheses ..

(+x^2+3x+x+3)-12=0

We get rid of parentheses

x^2+3x+x+3-12=0

We add all the numbers together, and all the variables

x^2+4x-9=0

a = 1; b = 4; c = -9;
Δ = b2-4ac
Δ = 42-4·1·(-9)
Δ = 52
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

The end solution:

$\sqrt{\Delta}=\sqrt{52}=\sqrt{4*13}=\sqrt{4}*\sqrt{13}=2\sqrt{13}$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(4)-2\sqrt{13}}{2*1}=\frac{-4-2\sqrt{13}}{2}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(4)+2\sqrt{13}}{2*1}=\frac{-4+2\sqrt{13}}{2}
$