what is the value of 81^-3/4 ?

(x^a)^b = x^(ab)
x^(-a) = 1/x^a

81^(-3/4)
= (3^4)^(-3/4)
= 3^(-3)
= 1/27

81^-3/4
= 4√(1/81^3) (4√ = extract the fourth root)
= 4√(1/531441)
= 1/27