0=10500-400x+3x^2

Solution for 0=10500-400x+3x^2 equation:

0=10500-400x+3x^2

We move all terms to the left:

0-(10500-400x+3x^2)=0

We add all the numbers together, and all the variables

-(10500-400x+3x^2)=0

We get rid of parentheses

-3x^2+400x-10500=0

a = -3; b = 400; c = -10500;
Δ = b2-4ac
Δ = 4002-4·(-3)·(-10500)
Δ = 34000
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:

$\sqrt{\Delta}=\sqrt{34000}=\sqrt{400*85}=\sqrt{400}*\sqrt{85}=20\sqrt{85}$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(400)-20\sqrt{85}}{2*-3}=\frac{-400-20\sqrt{85}}{-6}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(400)+20\sqrt{85}}{2*-3}=\frac{-400+20\sqrt{85}}{-6}
$