How would I get the "y" for -4 if the function was (1/3)^x and x is the power?

You take the given x-value, in this case -4, and insert it into the function.
$${\left({\frac{{\mathtt{1}}}{{\mathtt{3}}}}\right)}^{{\mathtt{x}}}$$ becomes $${\left({\frac{{\mathtt{1}}}{{\mathtt{3}}}}\right)}^{-{\mathtt{4}}}$$, and you get your answer, $${\left({\frac{{\mathtt{1}}}{{\mathtt{3}}}}\right)}^{-{\mathtt{4}}} = {\mathtt{81}}$$

Since the function is (1/3)^x , substitute -4 where the x appears, giving you (1/3)^-4.

I get 81.