Answer
$-2a^7y^2$
Work Step by Step
$\sqrt[3] {-8a^{21}y^6}=-2a^7y^2$
because,
$(-2a^7y^2)^3=-8a^{21}y^6$
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.