What was the hardest concept in mathematics for you to understand/learn. For me it was probably adding/subtracting negative and positive integers back in grade 8. That **** was so hard, but now that I look back I laugh at myself.
I HATED geometry. It was my first class in high school so having to deal with the adjustment didn't help, and to this day I still struggle with comprehending plane geometry even though I'll be in AP Calc next year.