How calculate total of rows from a subform in the mainform?

How calculate total of rows from a subform in the mainform?

Greetings.
I would like to calculate the total of the rows from my subform in my mainform.
It is a screen to add transactions, so I have:

Mainform with three fields: Transaction Type, Transaction Category, Transaction Amount

Subform where each row has: Item Description, Item Price, Item Qty

I would like that on each row added in the subform the field Transaction Amount in the mainform be updated with the total.

Thanks in advance.