How do I embed a Math formula?

[DMD]

I am trying to design a template for receipts with a math formula always there:

Subtotal, multiply times % of sales tax, then the grand total.

When I used Lotus WordPro this was fairly easy, but in msWord 2007, Vista I haven't figured it out. I've tried the "Insert" tab and equation. Also, the "Layout" tab with Formula. Formula works one time, but it's not there in the template, the next time I want to make a receipt. Thank you.