Chapter 1 - Some Basic Concepts of Arithmetic and Algebra - 1.5 - Use of Properties - Problem Set 1.5 - Page 33: 29

$7$

We start with the given expression: $-21+22-23+27+21-19$ We use the commutative property of addition to rearrange some of the terms in order to make it easier to add/subtract: $21-21+22-19+27-23$ We apply the associative property to group terms that are easy to add: $(21-21)+(22-19)+(27-23)$ We add/subtract inside each of the parentheses: $0+3+4$ We add from left to right: $3+4=7$