Answer
not a real number
Work Step by Step
We know that $\sqrt[4] -256$ is not a real number, because there is no number $a$ such that $a^{4}=-256$.
In general, the nth root of a, written as $\sqrt[n] a$, will not be a real number if $a$ (also known as the radicand) is negative and $n$ (also known as the index) is positive.
