7x*x=21x

Solution for 7x*x=21x equation:

7x*x=21x

We move all terms to the left:

7x*x-(21x)=0

We add all the numbers together, and all the variables

-21x+7x*x=0

Wy multiply elements

7x^2-21x=0

a = 7; b = -21; c = 0;
Δ = b2-4ac
Δ = -212-4·7·0
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*7}=\frac{0}{14}
=0
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-21)+21}{2*7}=\frac{42}{14}
=3
$