(x+11)(x+15)=320

Solution for (x+11)(x+15)=320 equation:

(x+11)(x+15)=320

We move all terms to the left:

(x+11)(x+15)-(320)=0

We multiply parentheses ..

(+x^2+15x+11x+165)-320=0

We get rid of parentheses

x^2+15x+11x+165-320=0

We add all the numbers together, and all the variables

x^2+26x-155=0

a = 1; b = 26; c = -155;
Δ = b2-4ac
Δ = 262-4·1·(-155)
Δ = 1296
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{1296}=36$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(26)-36}{2*1}=\frac{-62}{2}
=-31
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(26)+36}{2*1}=\frac{10}{2}
=5
$