@tomleslie your code was very helpfull , thank you very much ! I think I used it correctly and finally got the right results .
so to define the problem precisely :
I want to plot u2 , following function : ( which is periodic function )
but I have to define argument (eta) of the function from the following inequality (m=eta)
so what I did , i found values of m where inequality is correct ( actually i did it manually( using 2d plot) ? because when I was solving inequality answers were only negative)
and then used that values of m=eta for the first equation to make 3d plot of u2
I used the code you gave me and here is the final result . I think it has to be ok
Thank you very much again !