multiply php to fixed values

Hello

i need to find out how to make some calculation in php, i have an output table on a page that has values (prices) that need to be be multiplied and divided by set values.

for example

This value <?php echo $row_Recordset1['rental_price']; ?>

needs to be multiplied by a set value of 21.00 then show the results

i take it the set value (21.00) would be stored as a value then echoed into a table then you would do something like this

<?php echo $row_Recordset1['rental_price']; * echo_value?>

but each result needs to be shown as an equation

bit lost

thanks