Answer
(a.) $x^{7}$.
(b.) $y^{4}$.
Work Step by Step
(a.) $=\sqrt[3] {x^{21}}$
Rewrite as an exponential expression.
$=x^{\frac{21}{3}}$
Simplify the exponent.
$=x^{7}$.
(b.) $\sqrt[6] {y^{24}}$
Rewrite as an exponential expression.
$=y^{\frac{24}{6}}$
Simplify the exponent.
$=y^{4}$.