How to implement this with flexbox?

Question

css

You need to do what you see in the picture using flexbox. What is the best way to do this?

Answer 1

Here you have a container and two blocks, left and right, the right one is divided into two more.

<div>
  <div>1</div>
  <div>
    <div>2</div>
    <div>3</div>
  </div>
</div>

Answer 2

Alternatively:
https://jsfiddle.net/mfacenxp/1/
Just pick the sizes you need.