## Algebra 1

$36$
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: $36$