84=3u^2

Solution for 84=3u^2 equation:

84=3u^2

We move all terms to the left:

84-(3u^2)=0

a = -3; b = 0; c = +84;
Δ = b2-4ac
Δ = 02-4·(-3)·84
Δ = 1008
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$u_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$u_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

The end solution:

$\sqrt{\Delta}=\sqrt{1008}=\sqrt{144*7}=\sqrt{144}*\sqrt{7}=12\sqrt{7}$
$u_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(0)-12\sqrt{7}}{2*-3}=\frac{0-12\sqrt{7}}{-6}
=-\frac{12\sqrt{7}}{-6}
=-\frac{2\sqrt{7}}{-1}
$
$u_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(0)+12\sqrt{7}}{2*-3}=\frac{0+12\sqrt{7}}{-6}
=\frac{12\sqrt{7}}{-6}
=\frac{2\sqrt{7}}{-1}
$