R(x)=700-0.4x^2

Solution for R(x)=700-0.4x^2 equation:

(R)=700-0.4R^2

We move all terms to the left:

(R)-(700-0.4R^2)=0

We get rid of parentheses

0.4R^2+R-700=0

a = 0.4; b = 1; c = -700;
Δ = b2-4ac
Δ = 12-4·0.4·(-700)
Δ = 1121
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$R_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$R_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$R_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(1)-\sqrt{1121}}{2*0.4}=\frac{-1-\sqrt{1121}}{0.8}
$
$R_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(1)+\sqrt{1121}}{2*0.4}=\frac{-1+\sqrt{1121}}{0.8}
$