What number correctly completes the statement using the commutative property of addition? 5 + (-2.1) = - 2.1 + ?

The commutative property of addition states that a+b=b+a. In other words, when you're adding, the order in which you add doesn't matter. 2+3=5, and 3+2=5.

For your question, all that's changed is the values of a and b. a=5 and b=-2.1, so if we look at a+b=b+a and plug in our values of a and b, we get 5 + (-2.1) = - 2.1+5. The parentheses only appear on one side of the equation because having an addition and subtraction sybol right next to each other can be rather confusing: 5+-2.1, so adding the parentheses is like mathematical punctuation.