Question: Poisson equation algo help

hey friends..
Am trying to solve 2d poission equation with finite difference method and using different scheme is got

u_i-1,j -8u_i,j + u_i+1,j + 3u_i,j-1 +3u_i,j+1 = -h *16

and i have made code for this which is

> restart;
> with(LinearAlgebra);
> printf(`The lefthand endpoint on the X-axis is 0.\n`);
> OK := FALSE;
> while OK = FALSE do printf(`Input the righthand endpoint on the X-axis.\n`);

 FX := scanf(%f)[1];

if FX

else

 OK := TRUE

 end if

 end do;
> OK := FALSE;
> while OK = FALSE do printf(`Input the maximum value of the time variable Y.\n`); 

FY := scanf(%f)[1]; 

if FY

else 

OK := TRUE 

end if

 end do;
> OK := FALSE;
> while OK = FALSE do printf(`Input integer m = number of intervals on X-axis\n`);

 printf(`and N = number of time intervals - separated by a blank.\n`);

 printf(`Note that m must be 3 or larger.\n`);

 M := scanf(%d)[1];

 N := scanf(%d)[1];

if M

 else 

OK := TRUE 

end if

 end do;
> H := FX/M;
> # boundary condition given below
> for i while i < M do u[i, -1] := u[i, 1]-2*H*u[i, 0] end do;
> for i while i < M do u[i, N+1] := u[i, N-1]-2*H*u[i, N] end do;
> for j from 0 while j> for j from 0 while j

uptill here i have written the code but as am new user 

of maple and dont know much so please could anyone 

can help me in wirting further code for this

 i want to solve difference scheme that i wrote above.

this difference scheme would lead me to tri daigonal matrix 

which i have to solve by gauss elimination method

 or by any other method but how should i continue my algo that i dont know

 and boundary conditions are use where fictitious points comes..

will be waiting for your help guyz..