Question 678244
The number under a cube root it is not restricted to just non-negative numbers as for the one under the square root. 

∛(-64) = -4 because {{{(-4)^3 = -64}}}.

∛(64) = 4 because {{{4^3= 64}}}.

Thus {{{16-x^2}}} can be anything you want: negative, zero, or positive number. Which means you can plug any number you want for x, thus the domain is all real numbers.