In the first step, the associative property of addition was applied. This property states that:
(a + b) + c = a + (b + c)
Example
Suppose you want to compute the next addition:
3 + 1 + 6
You can add first 3 + 1
(3 + 1) + 6
4 + 6 = 10
or you can add first 1 + 6
3 + (1 + 6)
3 + 7 = 10
As you can see in this example, the result is the same. This is what the associative property states