A=200x+2.67x^2

Solution for A=200x+2.67x^2 equation:

=200A+2.67A^2

We move all terms to the left:

-(200A+2.67A^2)=0

We get rid of parentheses

-2.67A^2-200A=0

a = -2.67; b = -200; c = 0;
Δ = b2-4ac
Δ = -2002-4·(-2.67)·0
Δ = 40000
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$A_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$A_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{40000}=200$
$A_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-200)-200}{2*-2.67}=\frac{0}{-5.34}
=0
$
$A_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-200)+200}{2*-2.67}=\frac{400}{-5.34}
=-74+2.6304347826087/2.9021739130434
$