Question

What property justifies that 3y+9 = 9+3y

Answer 1

3y + 9 = 9 + 3y
Commutative Property

For addition, the rule is
a + b = b + a
in numbers, this means 2 + 3 = 3 + 2

Answer 2

What property justifies:

6a+(2a+8b)