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If I understood you correctly, you want to perform calculation inside of your form. You can use From calculation widget to perform calculation. Inside of this guide you can find how to do it: https://www.jotform.com/help/259-How-to-Perform-Calculation-in-the-Form
This guide can also help you to create your first form:
When you create your form you can add it to your Facebook page:
If you have some issue with setting calculation isnide of your form, please provide us ID of your form and provide us details about what you are trying to achieve and we will be happy to assist you.
Hope this will help. Let us know if you need further assistance.