Question 43346
You probably want the following:

81^(-1/4)

The negative means "invert"; the 1/4 means "take the 4th root"
These operations may be done in any order.

Inverting first you get (1/81)^(1/4)
Then taking the 4th root you get 1/3

Taking the 4th root first you get 3^(-1)
Then inverting you get 1/3

In either case you get 1/3 as the answer.
Cheers,
stan H.