C(x)=0.4x^2-80x+11,892.

Solution for C(x)=0.4x^2-80x+11,892. equation:

(C)=0.4C^2-80C+11.892.

We move all terms to the left:

(C)-(0.4C^2-80C+11.892.)=0

We get rid of parentheses

-0.4C^2+C+80C-11.892.=0

We add all the numbers together, and all the variables

-0.4C^2+81C=0

a = -0.4; b = 81; c = 0;
Δ = b2-4ac
Δ = 812-4·(-0.4)·0
Δ = 6561
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$C_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$C_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{6561}=81$
$C_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(81)-81}{2*-0.4}=\frac{-162}{-0.8}
=202+0.39999999999998/0.8
$
$C_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(81)+81}{2*-0.4}=\frac{0}{-0.8}
=0
$