Si potrebbe fare meglio cmq:

Codice PHP:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>Documento senza titolo</title>
<script language="javascript" type="text/javascript">
function q9_change(cosa){
if (cosa == "si")
document.getElementById("ac").style.visibility = "visible";
else
document.getElementById("ac").style.visibility = "hidden";
}
</script>

</head>

<body onload="q9_change('no')">
<table>
<tr>
<td width="5%"></td>
<td width="3%">Q5-</td>
<td width="92%">
Hai partecipato a corsi di educazione sessuale 
<input type="radio" name="q5" value="Si"  onclick="q9_change('si')" >Si 
<input type="radio" name="q5" value="No" onclick="q9_change('no')">No

</td>
</tr>
<tr id="ac">
<td width="5%"></td>
<td width="3%"></td>
<td width="92%">
- Se la risposta è SI, dove ? : 
<input name="q6" size="75"> 
</td>
</tr>

</table>
</body>
</html> 
Ciao
Scuba