<html>
<body>

<script type="text/javascript">
function UpdateStuff(value)
{
  x = value;
  y = 2*x + 12;

  document.getElementById('valx').innerHTML = x;

  document.getElementById('valy').innerHTML = y;
}
</script>

<form>
<select name="cars" onchange="UpdateStuff(this.value);">
  <option value="0" selected="selected"></option>
  <option value="1">1</option>
  <option value="2">2</option>
  <option value="3">3</option>
  <option value="4">4</option>
</select>
</form>

X: <span id="valx">0</span> <br />
Y: <span id="valy">0</span> <br />

</body>
</html>