As has been expressed by David Reiss there are a number of syntax errors. Please note that N is a special symbol in Mathematica and in general it is advisable not to capitalize your own functions to avoid such conflicts.

The following is your code rewritten (unfortunately I had to type as your post was an image so apologies for transcription errors):

a[n_] := (2 (-2 (Pi - 2) n^2 - 1) Sin[n Pi] +

Pi n)/(Sinh[n Pi] Pi^2 n^2 (4 n^2 - 1));

u[x_, y_, num_] :=

Sum[a[j] Sinh[j Pi x] Sin[j Pi y] + x - x y, {j, 1, num}]

Plotting:

Plot3D[Evaluate@u[x, y, 100], {x, 0, 1}, {y, 0, 1}]

yields: