Question: adding different matrix diagonally

Hi, I have two matrix



How do i make it into a [9x9] matrix? I is a diagonal matrix in the sequence of a,b,b,a. The first end of first "a" is added with begining of "b", the end of second "b" is added with the begining of third "b", and the end of third "b" is added with the beginning of final "a".

The answer that i'm getting shall be as follow :


 Please help. Thanks.

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