Chapter 1 - Introduction to Algebraic Expressions - 1.2 The Commutative, Associative, and Distributive Laws - 1.2 Exercise Set - Page 17: 35

$(2a)b$

The associative law of multiplication states that $a\times(b\times c)=(a\times b)\times c$ (where $a$, $b$, and $c$ are real numbers). That is, changing the grouping of numbers to be multiplied does not affect the answer. Therefore, $2(ab)=2\times(a\times b) =(2\times a)\times b=(2a)b$.