This works pretty well, and I can clearly see what you did, but as an alternative you didn't need any of the left right center classes, or 2 rows, you could have just used had a flex container with 6 divs inside wrapped at 30% and it would have made 2 rows automatically for you.
https://github.com/martincaddy/colmaracademy/blob/31e00c330dcc2e063ef104f790397cdb5026d738/index.html#L112-L198
This works pretty well, and I can clearly see what you did, but as an alternative you didn't need any of the left right center classes, or 2 rows, you could have just used had a flex container with 6 divs inside wrapped at 30% and it would have made 2 rows automatically for you.