Answer
$x^7y^7$
Work Step by Step
RECALL:
(i) $\sqrt[n]{a} = a^{\frac{1}{n}}$
(ii) $\sqrt[n]{a^m} = a^{\frac{m}{n}}$
(iii) $(ab)^m = a^mb^m$
(iv) $(a^m)^n=a^{mn}$
Use the rules above to obtain:
$=\left((xy)^{\frac{1}{3}}\right)^{21}
\\=(xy)^{\frac{1}{3}\cdot21}
\\=(xy)^7
\\=x^7y^7$
