How to create if and else statement?

Hello Friends,
It's been a while since I've worked with My Visual Database. Been working on my degree and finally complete it during this pandemic.
Now I'm back at it creating simple databases with My Visual Database.

I created a database for my monthly and annual expenses.
I would like to be able to automatically calculate my expenses based on the monthly and annual selected Combobox and have it reflect in the input box.

Attached is a screenshot of UI.

Thanks
Eddie