You can add the numbers in any order and your answer will come out the same. You can add the first two first or the second two first. You can even add the first and last first.
7 + 2 = 9
/ \
3 + 4 + 2 = 9
\ /
3 + 6 = 9
10 + 3 = 13
/ \
4 + 6 + 3 = 13
\ /
4 + 9 = 13