A Strange Case

Today, something strange has happen, it's about the math. homework given back by our teacher. When i check one of the question, i got it right, but when i see the method i do, i can't understand how i do, n also how can i think of that method, because it is impossible to do like that. I think about that question for almost half an hour, i also don't know how can i do that question by that method at the time i do a few weeks ago. Nevermind lah, juz forget about it.