Introductory Algebra for College Students (7th Edition)

$-5.7$.
The addends (numbers being added have) the same sign. To add two numbers with the same sign, perform the following steps: (1) Add the absolute values. $|-3.6|+|-2.1|=3.6+2.1=5.7$ (2) Use the sign of the numbers. $-3.6+(-2.1)=-5.7$ Thus, the sum is $-5.7$.