You missed the last instruction, which says that you have to use colspan property in the column of the first row of the table. The correct code will be:
<!doctype html>
<html>
<head>
<title>
Hello
</title>
</head>
<body>
<table border="1px" width="75%">
<tr height="50px">
<td colspan="4">
<input type="text" style="width:95%;height:50px;"/>
</td>
</tr>
<tr height="50px">
<td></td>
<td></td>
<td></td>
<td></td>
</tr>
</table>
</body>
</html>