(3.8-x)*(-4.2-x)=0

Solution for (3.8-x)*(-4.2-x)=0 equation:

(3.8-x)(-4.2-x)=0

We add all the numbers together, and all the variables

(-1x+3.8)(-1x-4.2)=0

We multiply parentheses ..

(+x^2+4.2x-3.8x-15.96)=0

We get rid of parentheses

x^2+4.2x-3.8x-15.96=0

We add all the numbers together, and all the variables

x^2+0.4x-15.96=0

a = 1; b = 0.4; c = -15.96;
Δ = b2-4ac
Δ = 0.42-4·1·(-15.96)
Δ = 64
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{64}=8$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(0.4)-8}{2*1}=\frac{-8.4}{2}
=-4+0.4/2
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(0.4)+8}{2*1}=\frac{7.6}{2}
=3+1/1.25
$