Answer
$\frac{1}{y^{21}}$
Work Step by Step
We are given the expression $(y^{7})^{-3}$.
To simplify, we can use the power rule, which holds that $(a^{m})^{n}=a^{m\times n}$ (where a is a real number, and m and n are integers).
$(y^{7})^{-3}=y^{7\times-3}=y^{-21}$
To simplify this into a positive exponent, we know that $a^{-n}=\frac{1}{a^{n}}$ (where a is a nonzero real number and n is a positive integer).
Therefore, $y^{-21}=\frac{1}{y^{21}}$
