If it is in absolute value:
[M, I] = max( abs( A(:,5) - A(:,6) ))
and otherwise remove the call to ABS.
PS: if you want to find all rows where the max occurs (if there can be multiple), you will have to do it as follows:
absDiff = abs( A(:,5) - A(:,6) ) ;
M = max( absDiff ) ;
I = find( absDiff == M ) ;
