I have no idea what you are talking about. Laplace's equation is
[tex]\nabla^2 \phi= 0[/tex]
I assume, since you say "triangular domain" that this is in two dimensions:
[tex]\frac{\partial^2\phi}{\partial x^2}+ \frac{\partial^2\phi}{\partial y^2}= 0[/tex]
but I have no idea what "M" and "N" are.
edit: from your next thread it appears you are talking about a finite difference numerical method and M and N are the number of mesh points. It would have helped if you had said that!
In any case, these both look like homework problems so I am going to move them there. Also, we need to know what you understand about these and where your difficulty is so you will need to show some work before we can help you.