JSFiddle - React, Tailwind, and code Playground

HTML

<select id="combo" class="combo">
    <option value="1">Test 1</option>
    <option value="2">Test 2</option>
    <option value="3">Test 3</option>
    <option value="6">Test 6</option>
    <option value="4">Test 4</option>
    <option value="5">Test 5</option>
</select>


<select id="combo1" class="combo">
    <option value="1">Test 1</option>
    <option value="2">Test 2</option>
    <option value="3">Test 3</option>
    <option value="6">Test 6</option>
    <option value="4">Test 4</option>
    <option value="a5">Test 5</option>
</select>

<br>
    <input type = "text" id = "firstvalue" value= "Test">
        <input type = "text" id = "firstvalue1" value= "Test">

JavaScript

$(document).ready(function() {
  
    $('.combo').change(function() {
        $('#firstvalue').val($('#combo').val() +"+"+$('#combo1').val()+"=");
        $('#firstvalue1').val(parseInt($('#combo').val()) +parseInt($('#combo1').val()));
    });
    
    
  
});