12*x=0.4*x^2

Solution for 12*x=0.4*x^2 equation:

12x=0.4x^2

We move all terms to the left:

12x-(0.4x^2)=0

We get rid of parentheses

-0.4x^2+12x=0

a = -0.4; b = 12; c = 0;
Δ = b2-4ac
Δ = 122-4·(-0.4)·0
Δ = 144
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{144}=12$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(12)-12}{2*-0.4}=\frac{-24}{-0.8}
=+30
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(12)+12}{2*-0.4}=\frac{0}{-0.8}
=0
$