Vectors in WordMat

I am using WordMat for everything in my Math-class, but when i try to calculate with Vectors, the program can not find a solution. I am trying to calculate something as simple as this:

A message pops up saying:
'Error. The expression could not be calculated. (%i11)'

Once i got an error-finding message saying that something is wrong with one of my definitions, and that it was something with 3_4. I did not insert any '_' in my definition though.

I have tried making the definitions using the design funktions in Word itself, and also by using the keyboard shortcuts. I have also tried writing 'a' and 'b' without the arrow above. I have also tried to just simply write the numbers in the calculation instead of defining a and b but still does not work.

I use WordMat in Microsoft Word 2011. I do have Microsoft Word 2016 installed, but 2011 is my standard word program, and what i use when i use WordMat.

My computer is a quite new Apple MacBook Pro and my operating system is OS X El-Capitan 10.11.6

I hope you can help my finding the solution.

Best regards

Livsen