thank u so much but still i tried to draw it but this error[Options expected (instead of Null) beyond position 3 in Plot3D[w[x,y],{x,-((3 [Pi])/2),(3 [Pi])/2},{y,-((3 [Pi])/2),(3 [Pi])/2},Mesh->{{-5},{{2.25,Directive[Thick,Red]}}},PlotStyle->LightBlue,Null,AxesLabel->{x,y,z}]. An option must be a rule or a list of rules. >>] appears sorry for bothering u because i m just a beginner and i study in german ...so complicated ..thanks anyway

What code do you have already?

First, try making the plot of w(x,y)=cosx.siny.

Then separately try to make the plot of your cuts for several different values.