Stuck in code should add a box-shadow property for the thumbnail id

i stuck in these where to add a box property ?? help please

h4 { text-align: center; background-color: rgba(45, 45, 45, 0.1); padding: 10px; font-size: 27px; } p { text-align: justify; } .links { text-align: left; color: black; } .fullCard { width: 245px; border: 1px solid #ccc; border-radius: 5px; margin: 10px 5px; padding: 4px; box-shadow: 0 10px 20px rgba(0,0,0,0.19), 0 6px 6px rgba(0,0,0,0.23); } .cardContent { padding: 10px; } .cardText { margin-bottom: 30px; }
<div class="cardText">

  <h4>Alphabet</h4>

  <hr>

  <p><em>Google was founded by Larry Page and Sergey Brin while they were <u>Ph.D. students</u> at <strong>Stanford University</strong>.</em></p>

</div>

<div class="cardLinks">

  <a href="https://en.wikipedia.org/wiki/Larry_Page" target="_blank" class="links">Larry Page</a><br><br>

  <a href="https://en.wikipedia.org/wiki/Sergey_Brin" target="_blank" class="links">Sergey Brin</a>

</div>

where is your thumbnail id ? , please be more specific so we can help