`1/81=b^-4` `81=b^4` `b=root(4)81=3`

`1/81=b^-4`

`81=b^4`

`b=root(4)81=3`

The first line follows from the logarithm definition. The second line uses negative exponents.

Then we find the 4th root of both sides.

Please support IntMath!