84=21b^2

Solution for 84=21b^2 equation:

84=21b^2

We move all terms to the left:

84-(21b^2)=0

a = -21; b = 0; c = +84;
Δ = b2-4ac
Δ = 02-4·(-21)·84
Δ = 7056
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$b_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$b_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{7056}=84$
$b_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(0)-84}{2*-21}=\frac{-84}{-42}
=+2
$
$b_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(0)+84}{2*-21}=\frac{84}{-42}
=-2
$