R=400x-0.01x^2

Solution for R=400x-0.01x^2 equation:

=400R-0.01R^2

We move all terms to the left:

-(400R-0.01R^2)=0

We get rid of parentheses

0.01R^2-400R=0

a = 0.01; b = -400; c = 0;
Δ = b2-4ac
Δ = -4002-4·0.01·0
Δ = 160000
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$R_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$R_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{160000}=400$
$R_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-400)-400}{2*0.01}=\frac{0}{0.02}
=0
$
$R_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-400)+400}{2*0.01}=\frac{800}{0.02}
=40000
$