tmp = A(1,:)-A(2,:)
But A1 is a 3x3 matrix and not like A which is a 2x2, so how do you want to insert the elements?
We could do it either
A1(2, 1:2)=tmp
or
A1(2, 2:3)=tmp
In the above expressions you may substitute tmp and write A(1,:)-A(2,:) directly if you wish.