1400=700x-x^2

Solution for 1400=700x-x^2 equation:

1400=700x-x^2

We move all terms to the left:

1400-(700x-x^2)=0

We get rid of parentheses

x^2-700x+1400=0

a = 1; b = -700; c = +1400;
Δ = b2-4ac
Δ = -7002-4·1·1400
Δ = 484400
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:

$\sqrt{\Delta}=\sqrt{484400}=\sqrt{400*1211}=\sqrt{400}*\sqrt{1211}=20\sqrt{1211}$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-700)-20\sqrt{1211}}{2*1}=\frac{700-20\sqrt{1211}}{2}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-700)+20\sqrt{1211}}{2*1}=\frac{700+20\sqrt{1211}}{2}
$