Answer
All real numbers, $x\ne+1,-1$
Work Step by Step
$$x^{4}-1=0$$
$$x^{4}=1$$
$$\sqrt[4] (x^{4})=\sqrt[4] 1$$
$$x=+1,-1$$
We have to be careful that we include -1 here.
You can help us out by revising, improving and updating this answer.
Update this answerAfter you claim an answer you’ll have 24 hours to send in a draft. An editor will review the submission and either publish your submission or provide feedback.