JSFiddle - React, Tailwind, and code Playground

HTML

<button id="roll" >Button</button>
<p>Rectangular Prism
    <p>Length:
        <input type="text" id='Length' size="3" />
        <br/>Width:
        <input type="text" id='Width' size="3" />

        <br/>Answer:
        <input type="text" id='Answer' size="3" oninput="RP();" />
    </p>
</p>

JavaScript

function RP() {
    var l = document.getElementById('Length').value;

    var w = document.getElementById('Width').value;


    var answer = parseInt(l) + parseInt(w);

    document.getElementById('Answer').value = answer;
}