I know I need to do something like this but I'm not sure how to trigger a check when the button is checked and to hide the "source_other" box when it is unchecked again. I'm also sure my code grammar is bad.

if(source("source_2").prop('checked', true)) {

      $('#source_describe').slideDown();

}

<form name="registration" action="" method="post">

      <div>How did you hear about us?</div>

      <div>

      <input type="radio" title="Facebook" name="source" id="source_1" class="radio1">

      <label for="Facebook">Facebook</label>

      </div>

      <div class="hide_this" id="source_describe">

            <label for="source_other">Other (describe)</label>

            <input type="text" title="Other, please describe." name="source_other" id="other_source" size="20">

      </div>

</form>