x(21-2x)=0

Solution for x(21-2x)=0 equation:

x(21-2x)=0

We add all the numbers together, and all the variables

x(-2x+21)=0

We multiply parentheses

-2x^2+21x=0

a = -2; b = 21; c = 0;
Δ = b2-4ac
Δ = 212-4·(-2)·0
Δ = 441
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{441}=21$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(21)-21}{2*-2}=\frac{-42}{-4}
=10+1/2
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(21)+21}{2*-2}=\frac{0}{-4}
=0
$