Evaluate the following. Click on "Not a real number" if applicable. (a) $\sqrt[4]{-81}=\square$ (b) $\sqrt[3]{-64}=\square$

Step 1 ：\(\sqrt[4]{-81}\) is not a real number because the fourth root of a negative number is not defined in the real number system.

Step 2 ：\(\sqrt[3]{-64} = -4\) because \(-4^3 = -64\)

Step 3 ：\(\boxed{-4}\) is the final answer for \(\sqrt[3]{-64}\)