(5-x)(8-x)-4=0

Solution for (5-x)(8-x)-4=0 equation:

(5-x)(8-x)-4=0

We add all the numbers together, and all the variables

(-1x+5)(-1x+8)-4=0

We multiply parentheses ..

(+x^2-8x-5x+40)-4=0

We get rid of parentheses

x^2-8x-5x+40-4=0

We add all the numbers together, and all the variables

x^2-13x+36=0

a = 1; b = -13; c = +36;
Δ = b2-4ac
Δ = -132-4·1·36
Δ = 25
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{25}=5$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-13)-5}{2*1}=\frac{8}{2}
=4
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-13)+5}{2*1}=\frac{18}{2}
=9
$