Answer
$-5.7$.
Work Step by Step
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 $-5.7$.
