Plotting Solutions Using Finite Difference Method with M = N = 20

[squenshl]

How do I use the finite difference method with M = N = 20 to obtain a plot of the solution of

$$\nabla$$²u = 1, 0 < x < 1, 0 < y < 1,
u(x,0) = x(1-x), u(x,1) = x(1-x), 0 $$\leq$$ y $$\leq$$ 1,
u(0,y) = 0, u(1,y) = 0, 0 $$\leq$$ y $$\leq$$ 1.

 

[HallsofIvy]

Okay, what have you done on this? In particular, what would $\nabla^2 u= 1[/math] look like in terms of this mesh?$