2100=(1000-100x)x

Solution for 2100=(1000-100x)x equation:

2100=(1000-100x)x

We move all terms to the left:

2100-((1000-100x)x)=0

We add all the numbers together, and all the variables

-((-100x+1000)x)+2100=0


We calculate terms in parentheses: -((-100x+1000)x), so:

(-100x+1000)x

We multiply parentheses

-100x^2+1000x

Back to the equation:

-(-100x^2+1000x)


We get rid of parentheses

100x^2-1000x+2100=0

a = 100; b = -1000; c = +2100;
Δ = b2-4ac
Δ = -10002-4·100·2100
Δ = 160000
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{160000}=400$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-1000)-400}{2*100}=\frac{600}{200}
=3
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-1000)+400}{2*100}=\frac{1400}{200}
=7
$