It would be a big problem for Tofugu if WaniKani's revenue were to drop.
I tried to solve this math question for thirty seconds and I couldn't figure it out, can you do it for me?
"Can you do me a favor?" "Sure, what is it?" "Can you look after my dog for the weekend?" "No problem. You can count on me."
The problem topic is the problem or question you have to answer. If you think about it, topics as problems tends to be a school-related thing. So, when you have to answer a problem in school, that's 問題.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.