Chapter 6 - Section 6.1 - The Greatest Common Factor and Factoring by Grouping - Exercise Set - Page 427: 56

$(x+5)(x+3)$

Factoring the $GCF= (x+5) ,$ the given expression, $ x(x+5)+3(x+5) ,$ is equivalent to \begin{array}{l}\require{cancel} (x+5) \left[ \dfrac{x(x+5)}{(x+5)}+\dfrac{3(x+5)}{(x+5)} \right] \\\\= (x+5) \left[ \dfrac{x\cancel{(x+5)}}{\cancel{(x+5)}}+\dfrac{3\cancel{(x+5)}}{\cancel{(x+5)}} \right] \\\\= (x+5)(x+3) .\end{array}