Chapter 1 - Introduction to Algebraic Expressions - 1.2 The Commutative, Associative, and Distributive Laws - 1.2 Exercise Set - Page 17: 36

$63r$

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, $9(7r)=9\times(7\times r) =(9\times 7)\times r=(63)r=63r$.