No surface appear when plotting a function with given constraints

Solution 1:

You define x1 and x2 as:

x1=linspace(0,2,50)
x2=linspace(0,2,50)

so each value of them is between 0 and 2. Then you edit them with:

x1, x2 = meshgrid(x1, x2)

At this point, x1 and x2 are no longer arrays, but matrices. Anyway, as before, each element within them is still between 0 and 2.
If you apply this filter:

f[-x1<=0] = nan

then each element of f becomes nan, since all elements of x1 are positive, so the expression -x1<=0 evaluates to False for every element of x1. The same logic applies for the filter:

f[-x2<=0] = nan

So, indeed, every element of f is nan and the plot is empty.
If you try to remove both of the above filters, then you get:

f[3*x1-x1*x2+4*x2-7<=0] = nan
f[2*x1+x2-3<=0] = nan
f[3*x1-4*x2**2-4*x2<=0] = nan