300=-0.2x^2+26x+500

Solution for 300=-0.2x^2+26x+500 equation:

300=-0.2x^2+26x+500

We move all terms to the left:

300-(-0.2x^2+26x+500)=0

We get rid of parentheses

0.2x^2-26x-500+300=0

We add all the numbers together, and all the variables

0.2x^2-26x-200=0

a = 0.2; b = -26; c = -200;
Δ = b2-4ac
Δ = -262-4·0.2·(-200)
Δ = 836
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:

$\sqrt{\Delta}=\sqrt{836}=\sqrt{4*209}=\sqrt{4}*\sqrt{209}=2\sqrt{209}$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-26)-2\sqrt{209}}{2*0.2}=\frac{26-2\sqrt{209}}{0.4}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-26)+2\sqrt{209}}{2*0.2}=\frac{26+2\sqrt{209}}{0.4}
$