Chapter 1 - Section 1.2 - Operations with Real Numbers and Simplifying Algebraic Expressions - Exercise Set - Page 29: 152

To add two numbers with the same sign, add their absolute values. The sum will have the same sign as the two numbers.

To add two numbers with the same sign, add their absolute values. The sum will have the same sign as the two numbers. Example: $-3+(-4)$ Add their absolute values: $|-3| + |-4| = 3+4 =7$ The sum must be negative, thus: $-3+(-4)= -7$