Answer:
Associative property of addition.
Explanation:
This looks to be the associative property of addition. Pretty sure.
This was explained to me as:
1 + (2 + 4) = (1 + 2 ) + 4
or
A + (B + C) = (A + B) + C
For further reference on addition properties note:
Commutative property is A + B = B + A
Identity property is A + 0 = A