0=-15+10x-0.2x^2

Solution for 0=-15+10x-0.2x^2 equation:

0=-15+10x-0.2x^2

We move all terms to the left:

0-(-15+10x-0.2x^2)=0

We add all the numbers together, and all the variables

-(-15+10x-0.2x^2)=0

We get rid of parentheses

0.2x^2-10x+15=0

a = 0.2; b = -10; c = +15;
Δ = b2-4ac
Δ = -102-4·0.2·15
Δ = 88
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

The end solution:

$\sqrt{\Delta}=\sqrt{88}=\sqrt{4*22}=\sqrt{4}*\sqrt{22}=2\sqrt{22}$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-10)-2\sqrt{22}}{2*0.2}=\frac{10-2\sqrt{22}}{0.4}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-10)+2\sqrt{22}}{2*0.2}=\frac{10+2\sqrt{22}}{0.4}
$