# Chapter 1 - Some Basic Concepts of Arithmetic and Algebra - 1.3 - Integers: Addition and Subtraction - Problem Set 1.3 - Page 20: 110

$=12$

#### Work Step by Step

Recall the following rule: $-a+b=b-a$ Thus, we obtain: $= - 9 + 21$ $= 21 - 9$ $= 12$

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