3=-0.1*x^2+9

Solution for 3=-0.1*x^2+9 equation:

3=-0.1x^2+9

We move all terms to the left:

3-(-0.1x^2+9)=0

We get rid of parentheses

0.1x^2-9+3=0

We add all the numbers together, and all the variables

0.1x^2-6=0

a = 0.1; b = 0; c = -6;
Δ = b2-4ac
Δ = 02-4·0.1·(-6)
Δ = 2.4
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{-(0)-\sqrt{2.4}}{2*0.1}=\frac{0-\sqrt{2.4}}{0.2}
=-\frac{\sqrt{}}{0.2}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(0)+\sqrt{2.4}}{2*0.1}=\frac{0+\sqrt{2.4}}{0.2}
=\frac{\sqrt{}}{0.2}
$