Chapter 1 - Section 1.2 - Operations with Real Numbers and Simplifying Algebraic Expressions - Exercise Set - Page 27: 16

$-9$

The addends have different signs, so to add them, perform the following steps: (1) Subtract the smaller absolute value from the bigger absolute value. $|-15|-|6| = 15-6 = 9$ (2) The sum will have the same sign as the number with the bigger absolute value. Thus, $-15+6=-9$