How to create equal height div irrespective of content in it

[vishaalmandge]

Hello, I'm trying to make equal height div using display: flex, but it won't work. How to do this. I want these cols equal height

[glavrjk]

<style type="text/css">
            .row-flex {
                display: flex;
                display: -webkit-flex;
                flex-wrap: wrap;
            }
</style>
<div class="row-flex">
***************
          <div class="col-xs-12"></div>
          <div class="col-xs-6"></div>
          <div class="col-xs-6"></div>
**************
</div>