Answer
$4$
Work Step by Step
$x^{-4}=\dfrac {1}{256}=\dfrac {1}{4^{4}}=4^{-4}\Rightarrow x=4$
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.