Subtract 2 different tables

[Larry Dobbs]

I have included how I can sub from 2different tables. Now I want to subtract TOTAL PAYMENTS from TOTAL PRINCIPAL.

Total Principal
<?php
$query = "SELECT sum(totalprincipal) AS totalprincipal FROM rentals,inventory where rentals_cst_id='$id' AND subid='$subid' AND rentals_inv_id = num AND rentals_active = 'Yes'";
$result = mysql_query($query) or die(mysql_error());
while($row = mysql_fetch_array($result)){
echo "$". $row['totalprincipal']; }?>

------------------------------

Total Payments:
<?php
$query = "SELECT sum(principal) AS totalpayments FROM payments where payments_id='$id' AND payments_subid='$subid' AND del='No'";
$result = mysql_query($query) or die(mysql_error());
while($row = mysql_fetch_array($result)){
echo "$". $row['totalpayments']; }?>

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I found a similar code but cannot seem to get it to work, here is what I have started with errors:

Balance Due:
<?php
$query = "SELECT tmp.subid, sum(tmp.totaldue) AS balancedue FROM [ SELECT rentals.subid, sum(totalprincipal) as totaldue from rentals group by rentals.subid UNION select payments.payments_subid, - sum(principal) as totaldue from payments group by payments.payments_subid ] as tmp group by tmp.subid";
$result = mysql_query($query) or die(mysql_error());
while($row = mysql_fetch_array($result)){
echo "$". $row['tmp']; }?>




Thanks.