Question: How to plot with variables

Hello everyone 


I have the following issue :

I want to create a different plots (2d/3d) with varying variables ( I mean expression contains different variables and I am changing their values in order to get different results ) 


for example: 




plot3d(u, x = -2 .. 2, y = -2 .. 2)

and I am getting an error 


could you please tell me how to plot in that way , that first i define the function with unknown variables , then i am giving them different values ( idea is that i have lots of variables and lots of ranges so it will not be convenient to change them manually into the expression ) and at the end to have a plot 

Thank you very  very much 

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