hello! so, I've came accross a problem I can't solve at all :c. The problem is as it follows:

**The constant term of **

I searched how to solve it and found only one solution, but I didn't understand what was done... they turned

into

and somehow found that k = 2. also, they found "

", which give us a = -3.

That's actually the right answer, but I can't figure out what was done to solve this problem... :c Can you guys explain it to me?

Thanks c: