21=7x2

Solution for 21=7x2 equation:

21=7x^2

We move all terms to the left:

21-(7x^2)=0

a = -7; b = 0; c = +21;
Δ = b2-4ac
Δ = 02-4·(-7)·21
Δ = 588
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:

$\sqrt{\Delta}=\sqrt{588}=\sqrt{196*3}=\sqrt{196}*\sqrt{3}=14\sqrt{3}$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(0)-14\sqrt{3}}{2*-7}=\frac{0-14\sqrt{3}}{-14}
=-\frac{14\sqrt{3}}{-14}
=-\frac{\sqrt{3}}{-1}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(0)+14\sqrt{3}}{2*-7}=\frac{0+14\sqrt{3}}{-14}
=\frac{14\sqrt{3}}{-14}
=\frac{\sqrt{3}}{-1}
$