How to delete the last rows and columns?

How to delete the last three rows and three columns from the following matrix without change N and then multiply the last off diagonal by 6?

then how can we store the new matrix?

M = zeros(N,N);
>> M(  1:1+N:N*N) = a;
>> M(N+1:1+N:N*N) = b;
>> M(  2:1+N:N*N-N) = c
M =
  -1   4   0   0   0   0   0   0   0   0
   2  -1   4   0   0   0   0   0   0   0
   0   2  -1   4   0   0   0   0   0   0
   0   0   2  -1   4   0   0   0   0   0
   0   0   0   2  -1   4   0   0   0   0
   0   0   0   0   2  -1   4   0   0   0
   0   0   0   0   0   2  -1   4   0   0
   0   0   0   0   0   0   2  -1   4   0
   0   0   0   0   0   0   0   2  -1   4
   0   0   0   0   0   0   0   0   2  -1

[darova]

What about this?

[Adam]

What is 'the last' off diagonal?