Answer:
Since your problem is x^4 = 81, what you have to undeo is the "to the fourth power" part.
The inverse (opposite operation) of "to the fourth power" is "to the fourth root".
(x^4) ^ 1/4 = 81 ^ 1/4
81 = 3^4
so, (3^4) ^ 1/4 = 3 ^ 1 = 3
by hand: (3^4) 1/4 = 3 ^
(4 × 1/4) = 3^1 = 3
(when you take a power to a power, multiply the exponents.)
Or, do it by calculator. One way of finding the fourth the fourth root is to use ^ (1/4).