Chapter 1 - Introduction to Algebraic Expressions - 1.2 The Commutative, Associative, and Distributive Laws - 1.2 Exercise Set - Page 17: 31

$ab+(c+d)$

The associative law of addition states that $a+(b+c)=(a+b)+c$ (where $a$, $b$, and $c$ are real numbers). That is, changing the grouping of numbers to be added does not affect the answer. Therefore, $(ab+c)+d=ab+(c+d)$.