3x*(7-x)=0

Solution for 3x*(7-x)=0 equation:

3x(7-x)=0

We add all the numbers together, and all the variables

3x(-1x+7)=0

We multiply parentheses

-3x^2+21x=0

a = -3; b = 21; c = 0;
Δ = b2-4ac
Δ = 212-4·(-3)·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*-3}=\frac{-42}{-6}
=+7
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(21)+21}{2*-3}=\frac{0}{-6}
=0
$