How do i add a second class attribute to my .key class

How do i add the second class to my existing .key element.

  **Your code so far**
/* file: index.html */
<!DOCTYPE html>
<html>
<head>
  <meta charset="UTF-8" />
  <title>Piano</title>
  <meta name="viewport" content="width=device-width, initial-scale=1.0" />
</head>
<body>
  <div id="piano">
    <div class="keys">
      <div class="key" class="black--key"></div>
      <div class="key"></div>
      <div class="key"></div>
      <div class="key"></div>
      <div class="key"></div>
      <div class="key"></div>
      <div class="key"></div>
    </div>
  </div>
</body>
</html>
/* file: styles.css */

  **Your browser information:**

User Agent is: Mozilla/5.0 (Linux; Android 8.0.0; SM-G935U) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/103.0.0.0 Mobile Safari/537.36

Challenge: Step 6

Link to the challenge:

you can add multiple classes by separating them with a space, like this: class="class1 class2"