HTML-Code: 
<script type="text/javascript">
function rechne()
{
  var x = document.getElementById("x-input").value;
  
  if(x > 100 || x < 1)
  {
    var out = "Fehler!";
  } else
  {
    var out = 3.5 * (x*x) + 70 * x + 500;
  }
  document.getElementById("out").value = out;
}
</script>

<input id="x-input" /><br />
<input type="button" value="Berechnen" onclick="rechne();" /><br />
<input id="out" readonly="readonly" />
So in etwa?