minor problems to fix in your query:
-The correct syntax is '>=', 'Greater than or equal to'.
-Any where condition after the first is proceeded by 'AND', not another 'WHERE'
You might also want to consider escaping your variables before use in the query.
<?
//corrected query with escaped value
$id = mysql_real_escape_string($_GET['id']);
$query_b= "SELECT AMOUNT FROM house_offers" .
"WHERE AMOUNT >= '$b3' " .
"AND AMOUNT < '$a3' AND HOUSEID='".$id."' " ;
$resultsb= mysql_query($query_b) or die("Query failed (): " . mysql_error());
$b4=@mysql_num_rows($resultsb);
?>