#### Answer

$36$

#### Work Step by Step

We start with the given expression: $21+6+9$
The commutative property of addition states that changing the order of the terms does not affect the sum.
We use the commutative property to rearrange the order: $21+9+6$
The associative property of addition states that changing the grouping of the terms does not affect the sum.
We use the associative property to group two of the addends: $(21+9)+6$
We now simplify using the order of operations:
First, we simplify inside the parentheses:$30+6$
Finally, we add: $36$