Chapter 9 - Section 9.7 - Common Logarithms, Natural Logarithms, and Change of Base - Vocabulary, Readiness & Video Check - Page 584: 1

10

A common logarithm is a logarithm to base 10. As shown on page 580, when a common logarithm is written in the form $log (x)$, this is equivalent to the expression $log_{10}x$. Therefore, $log 7$ is a common logarithm that can also be written as $log_{10}7$. So the base of $log 7$ is 10.