a = 15;
b = 7.5;
k = 4*100/pi; % Define constants.
X = linspace(-5,5,101);
[x,y] = meshgrid(X,X); % x and y range.
n = 1:2:199;
for p = 1:length(n)
u(:,:,p) = (sin(n(p)*pi*x/a).*sinh(n(p)*pi*y/a))./(n(p)*sinh(n(p)*pi*b/a)); % Distribution over all values of n.
end
u = k.*sum(u,3);
figure(1)
surf(x,y,u) % Surface plot.
figure(2)
contour(x,y,u) % Contour plot.
This produces the surface plot and contour plot you might be looking for. I have choosen an arbitrary range for x and y, and made sure a and b satisfy the conditions you specified.
