The answer is 1/3. The fourth root of 81 is 3 ($${\mathtt{3}}{\mathtt{\,\times\,}}{\mathtt{3}}{\mathtt{\,\times\,}}{\mathtt{3}}{\mathtt{\,\times\,}}{\mathtt{3}} = {\mathtt{81}}$$). When you take something to the negative first power, you take it's inverse $${\left({\frac{{\mathtt{a}}}{{\mathtt{b}}}}\right)}^{-{\mathtt{1}}} = {\frac{{\mathtt{b}}}{{\mathtt{a}}}}$$. Thus 3 becomes 1/3.
