If you mean sqrt(81)x^4 then:
+-9x^4
+9x^4 or -9x^4
(both + and - are true)
.
If you mean sqrt(81x^4) then:
+-9x^2
+9x^2 or -9x^2
(both + and - are true)
If you mean (sqrt(81x))^4 then:
( (81x)^(1/2) )^4
(81x)^2
6561x^2
.
If you mean fourth root(81x) then: (there's a little 4 on the radical sign)
(81x)^(1/4)
+-3x^4
+3x^4 or -3x^4
(+ and - both true)