Chapter 3 - Section 3.7 - Graphing Linear Inequalities - Exercise Set - Page 188: 64

$\dfrac{4}{49}$

Using the concepts of exponents, the given expression, $ \left( \dfrac{2}{7} \right)^2 ,$ is equivalent to \begin{array}{l}\require{cancel} \left( \dfrac{2}{7} \right)\left( \dfrac{2}{7} \right) \\\\= \dfrac{4}{49} .\end{array}