Very nicely done congratulations! I see you weren't able to place the quotations image on the first card. I had the same problem, eventually I set it as a background image, with no repeat property, and used background-position to position it the right way. So I would suggest that.
Really nicely done. It appears that in the midst of reading about those columns you forgot to vertically center the entire thing, but it's probably a quick fix. Besides that looks great to me.
For the box-shadow I recommend watching Kevin Powell's videos on the topic, or if you prefer there are multiple box-shadow generator websites that can help you. Besides that your solutions seems pretty solid, besides some minor issues. The thing that I would pay more attention to would be more semantic HTML, that is, try to replace the div tags with more semantic ones like main, article, etc. Also to change the image I recommend using the picture tag which you can learn about on this website.
The main problem that I see here is that the card in not centered vertically. To do it, you can use flexbox on the body and then center it. Additionally, I suggest to change the background color attribute from the ".preperation" class to "body" to have the whole page covered.
Honestly, seems like a perfect solution, except for the fact that for some reason it's not centered vertically. Probably an easy fix, but if you need help with that let me know (multiple ways to do that, eg. flexbox, grid, margin: auto, etc.).
To make the HTML more semantic and more concise, I would suggest to change <div class="main"></div> to <main></main>, and <p><b>text</b></p> with <h1>text</h1>.