Certainly I hope to see an addition, as in the end of this question says 'a11+a22'..
Still not very clear.
The following shows adding two matrices you have shown:
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First matrix is located in A2:B3, the second in D2:E3, and the result matrix obtained by addition in G2:H3.
In G2 enter and copy down:
=A2+D2
In H2 enter and copy down:
=B2+E2
If you mean by adding 'a11+a22' the sum of G2 and H3 of the third matrix wit as result 9...
Control+shift+enter, not just enter:
=SUM(N(OFFSET(G2,ROW(INDIRECT("1:2"))-1,ROW(INDIRECT("1:2"))-1)))
which is derived from the link I quoted earlier.