Chapter 5 - Cumulative Review - Page 334: 40b

$\dfrac{1}{a^4}$

Using laws of exponents, then, \begin{array}{l} \left( \dfrac{a^{-6}}{a^{-8}} \right)^{-2} \\\\= \left( a^{-6-(-8)} \right)^{-2} \\\\= \left( a^{2} \right)^{-2} \\\\= a^{-4} \\\\= \dfrac{1}{a^4} .\end{array}