Answer:
The correct options are:
![\begin{gathered} f(x)=4\sqrt[9]{x} \\ . \\ f(x)=-18\sqrt[3]{x} \end{gathered}](https://img.qammunity.org/2023/formulas/mathematics/college/4xdj9oum5t9it5qdh5v7ykqwflhg485ujv.png)
Step-by-step explanation:
A power function is a function that has the variable to the power of some fixed real number. A power function can't have the variable (x, in this problem) in the exponent.
Then, the only two option that follow this are:
![\begin{gathered} f(x)=4\sqrt[9]{x} \\ . \\ f(x)=-18\sqrt[3]{x} \end{gathered}](https://img.qammunity.org/2023/formulas/mathematics/college/4xdj9oum5t9it5qdh5v7ykqwflhg485ujv.png)
Because we can rewrite the exponent as:
![\begin{gathered} f(x)=4\sqrt[9]{x}=4x^{(1)/(9)} \\ . \\ f(x)=-18\sqrt[3]{x}=-18x^{(1)/(3)} \end{gathered}](https://img.qammunity.org/2023/formulas/mathematics/college/unirlvcks6e6ad024ufs593gw1iwng8eni.png)
Which is clearly, the variable to the power of a real number.