The brackets containing 7 have moved. That is, 7 is associated with something different on the left and right sides of the equation. The property that lets you do that is ...
... associative property
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Either way you evaluate the equation, it comes out
... 3 + 3 = 10 - 4
... 6 = 6
The appropriate choice is ...
... 6 . . . . . the second selection
