(x-3)(x+0.4)=0

Solution for (x-3)(x+0.4)=0 equation:

(x-3)(x+0.4)=0

We multiply parentheses ..

(+x^2+0.4x-3x-1.2)=0

We get rid of parentheses

x^2+0.4x-3x-1.2=0

We add all the numbers together, and all the variables

x^2-2.6x-1.2=0

a = 1; b = -2.6; c = -1.2;
Δ = b2-4ac
Δ = -2.62-4·1·(-1.2)
Δ = 11.56
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{-(-2.6)-\sqrt{11.56}}{2*1}=\frac{2.6-\sqrt{11.56}}{2}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-2.6)+\sqrt{11.56}}{2*1}=\frac{2.6+\sqrt{11.56}}{2}
$