JSFiddle - React, Tailwind, and code Playground

HTML

<div id="div1">
    u hf;qwhf;iwe;fjq;ejfiwq
</div>

<div id="div2">
    eoif qhewihfqjh fjqwefi qwef
</div>

<div id="div3">
    wpioq feqijwf[ qjepfqj
</div>

CSS

#div1
{
 height:200px;   
}

#div2
{
 height:150px;   
}

*{ border:1px solid black;}

JavaScript

onload=function() {

var lengthnews1 = ((document.getElementById("div1").offsetHeight)+"px");
var lengthnews2 = ((document.getElementById("div2").offsetHeight)+"px");

var x = document.getElementById("div3");
x.height = (lengthnews1 + lengthnews2); 

}