Chapter 9 - Roots and Radicals - 9.2 - Simplifying Radicals - Problem Set 9.2 - Page 406: 51

$-\dfrac{26a^4}{3}$

Using the properties of radicals, the given expression, $ -\dfrac{2}{3}\sqrt[]{169a^8} ,$ simplifies to \begin{array}{l}\require{cancel} -\dfrac{2}{3}\sqrt[]{(13a^4)^2} \\\\= -\dfrac{2}{3}\cdot13a^4 \\\\= -\dfrac{26a^4}{3} .\end{array} Note that all of the variables are assumed to represent non-negative real numbers.