Chapter 6 - Section 6.1 - The Greatest Common Factor and Factoring by Grouping - Exercise Set - Page 427: 7

$3y$

The $GCF$ of $ 9,18, $ and $ -3 $ is $ 3 .$ The $GCF$ of $ y^5,y^2, $ and $ y $ is $ y .$ Hence, the $GCF$ of the given expressions, $ 9y^5,18y^2, \text{ and } -3y ,$ is \begin{array}{l}\require{cancel} 3y .\end{array}