Go into Manage Database
Change the Total Price field type to a calculation field
= YourTable::quantity * YourTable::total price
Do not allow this field to be entered in edit mode from the field options on the layout.
As easy as that!
oh boy that was really simple. Thank you so much Lee.
Now I have another problem with the total price. it should write as 3.50 but it only writes 3.5 how do I change that?
Go to your layout
Click Edit Layout
On your layout click on the field in question
Open the inspector (click on the i/information symbol on the darkest grey toolbar)
Click on the Data tab
Scroll down to Data Formatting
Format as a decimal
Set decimal places to 2
"Do not allow this field to be entered in edit mode from the field options on the layout."
Calculation fields will not allow the user to edit their values so this precaution is not needed for calculation fields. If the field is of type number with an auto-entered calculation, then this may be needed--depending on the options selected for the auto-enter calculation.