Filtering with merged cells

I wonder if anyone can help...

I'm trying to filter a spread sheet I am currently working on but cant quite figure out how to format the cells correctly

For example, in Column A I have a list of Subjects, and in Column B is a list of jobs that need doing for each subject. I want to be able to merge the cells for each individual subject in A so that when I filter each one separately they still bring up all the relevant rows from B. 

I have already tried using the Find & Select / Go to special to fill in all of the cells and then using format painter over the cells I want. This work fine, however when I need to add an extra row I am having to reformat the whole document again.

Does anyone know any alternative ways for filtering with merged cells?