Explanation: (6 + 6) : 3 =
First work out the Brackets:
(6 + 6) : 3 = 12 : 3 =4 (Notice that the Brackets have gone now, as they have no meaning any more. This just between brackets. And between you and me.)
That's why the good answer is 4!
FINALLY: Try to understand the meaning of the next slide!