[![Problem](https://i.stack.imgur.com/UNwwb.jpg)](https://i.stack.imgur.com/UNwwb.jpg)
I was shown the above problem in my calculus class today. It seems that one can solve the limit in white by realizing that it mirrors the limit in yellow.

However, I don't understand what the exact relationship between those two limits is. My TA went over it rather quickly. Can someone explain what point he was trying to make?