Designing FlashCards using Flexbox Layout

Flexbox Layout was a difficult layout when I was reading about it and it is maintaining consistency, still being a difficult one while I try to build it.

I am making a simple Flashcard Quiz using HTML, CSS, and maybe minimal Javascript. Well, I did realize along the course, designing is as hard as coding especially when it contains math... 😣

My Design is a 3*3 Square Matrix, which I am absolutely failing to create :(

My Trial attempt looks like :-/

My trial attempt is here https://codepen.io/hennasingh04/pen/zYPpJLB

I will work some more on this tomorrow. If you have any suggestions, am eager to know… :D