20=7*x^2

Solution for 20=7*x^2 equation:

20=7x^2

We move all terms to the left:

20-(7x^2)=0

a = -7; b = 0; c = +20;
Δ = b2-4ac
Δ = 02-4·(-7)·20
Δ = 560
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:

$\sqrt{\Delta}=\sqrt{560}=\sqrt{16*35}=\sqrt{16}*\sqrt{35}=4\sqrt{35}$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(0)-4\sqrt{35}}{2*-7}=\frac{0-4\sqrt{35}}{-14}
=-\frac{4\sqrt{35}}{-14}
=-\frac{2\sqrt{35}}{-7}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(0)+4\sqrt{35}}{2*-7}=\frac{0+4\sqrt{35}}{-14}
=\frac{4\sqrt{35}}{-14}
=\frac{2\sqrt{35}}{-7}
$