Hallo, help please!

I am trying to create a robust feedback form for students where there are 2 marks (set up as dropdown lists because there are limited numbers you are allowed to use) and then the need for these marks to be averaged for the overall score.

I have tried to do this as a formula but either get no output or an error message. I have read up a bit on macros but increasingly confused!

Can anyone help me do this?

Details: I'm using Word for Mac 2016

Thanks...