Question

What property was used to rewrite the polynomial expression

(x+5)+3(a+(6x)y) to (5+x)+3(a+(6x)y)

Answer 1

commutative property because all that was changed was the 5+x they were flip flopped

Answer 2

In order to rewrite the polynomial expression (x+5)+3(a+(6x)y) into (5+x)+3(a+(6x)y) the commutative property of addition is used.
The commutative property is most commonly used property in maths. According the commutative property of addition changing the order of addends does not change the sum. Which means that x+5=5+x, and because of this :
(x+5)+3(a+(6x)y) = (5+x)+3(a+(6x)y)