6=-0.004x^2+1.2x+5

Solution for 6=-0.004x^2+1.2x+5 equation:

6=-0.004x^2+1.2x+5

We move all terms to the left:

6-(-0.004x^2+1.2x+5)=0

We get rid of parentheses

0.004x^2-1.2x-5+6=0

We add all the numbers together, and all the variables

0.004x^2-1.2x+1=0

a = 0.004; b = -1.2; c = +1;
Δ = b2-4ac
Δ = -1.22-4·0.004·1
Δ = 1.424
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{-(-1.2)-\sqrt{1.424}}{2*0.004}=\frac{1.2-\sqrt{1.424}}{0.008}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-1.2)+\sqrt{1.424}}{2*0.004}=\frac{1.2+\sqrt{1.424}}{0.008}
$