a:1$ b:1$ c:1$

u[m,n]:=sin(m*%pi*x/a)*sin(n*%pi*y/b);

plot3d(u[1,1],[x,0,a],[y,0,b])$

plot3d(u[2,3],[x,0,a],[y,0,b])$

contour_plot(u[1,1],[x,0,a],[y,0,b],
[legend,false],[grid, 200, 200],
[gnuplot_preamble,"set cntrparam levels 12"])$

contour_plot(u[2,3],[x,0,a],[y,0,b],
[legend,false],[grid, 200, 200],
[gnuplot_preamble,"set cntrparam levels 12"])$