64=81d^2

Solution for 64=81d^2 equation:

64=81d^2

We move all terms to the left:

64-(81d^2)=0

a = -81; b = 0; c = +64;
Δ = b2-4ac
Δ = 02-4·(-81)·64
Δ = 20736
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$d_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$d_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{20736}=144$
$d_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(0)-144}{2*-81}=\frac{-144}{-162}
=8/9
$
$d_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(0)+144}{2*-81}=\frac{144}{-162}
=-8/9
$