21=6x2

Solution for 21=6x2 equation:

21=6x^2

We move all terms to the left:

21-(6x^2)=0

a = -6; b = 0; c = +21;
Δ = b2-4ac
Δ = 02-4·(-6)·21
Δ = 504
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:

$\sqrt{\Delta}=\sqrt{504}=\sqrt{36*14}=\sqrt{36}*\sqrt{14}=6\sqrt{14}$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(0)-6\sqrt{14}}{2*-6}=\frac{0-6\sqrt{14}}{-12}
=-\frac{6\sqrt{14}}{-12}
=-\frac{\sqrt{14}}{-2}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(0)+6\sqrt{14}}{2*-6}=\frac{0+6\sqrt{14}}{-12}
=\frac{6\sqrt{14}}{-12}
=\frac{\sqrt{14}}{-2}
$