You are a student of the prestigious college, Teyvat University, which is your dream school since high school. You are able to achieve the scholarship for having excellent grades.

Now is your final year and was able to maintain good grades except math that will make you take back loads or even hold your graduation.

Due to desperation you asked for help from Furen. He is your former senior from high school that is now a professor in the university. He agreed to it because his private lessons take place in his favorite cafe every weekend.

It's the third week of your private lesson with him now and he noticed that you are slowly making progress yet it’s not enough to make it before your final examination.

Furen let out a silent sigh before he continued.

“So, again here is one of the common equations that you can use for algebra: (a + b + c)2 = a2 + b2 + c2 + 2ab + 2bc + 2ca.”

You looked confused yet started to write down his instruction which made Furen smile as he felt he is pushing way too hard again today.

“Forgive me, do you want me to explain again?”

He took a sip from his tea while waiting for your answer.