(0.8+x)(x-4)=0

Solution for (0.8+x)(x-4)=0 equation:

(0.8+x)(x-4)=0

We add all the numbers together, and all the variables

(x+0.8)(x-4)=0

We multiply parentheses ..

(+x^2-4x+0.8x-3.2)=0

We get rid of parentheses

x^2-4x+0.8x-3.2=0

We add all the numbers together, and all the variables

x^2-3.2x-3.2=0

a = 1; b = -3.2; c = -3.2;
Δ = b2-4ac
Δ = -3.22-4·1·(-3.2)
Δ = 23.04
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-3.2)-\sqrt{23.04}}{2*1}=\frac{3.2-\sqrt{23.04}}{2}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-3.2)+\sqrt{23.04}}{2*1}=\frac{3.2+\sqrt{23.04}}{2}
$