My knowledge of logarithms is a bit rusty but I think you can say that

\(b^{2/3}=81\).

Now to solve for \(b\) we need to take the square root of both sides and then cube it, giving you

\(b^{2/3\cdot3/2}=81^{3/2}\)

The exponent of b will be just 1, so now we have

\(b=729.\)

.