[x,y,x] = meshgrid(-10:0.1:10);
Correct code is important in MATLAB. Else it will not know that you really intended to write [x,y,z] and not [x,y,x].
Next, what do you think this does?
ineq = [0<=y, y<=3, 9-y.^2<=x, x<=9, 0<=z, z<=9-x]
It does not create something that is "and"ed across those inequaities. Instead, it creates a larger array.
Next, is the result of that operation actually a surface, or just something that you want to interpret as a surface?
So how can surf possibly interpret what it will see as a set of scattered points, some of which are NaNs, and know what you intend?
You MIGHT try this:
[x,y,z] = meshgrid(-10:0.1:10,-10:0.1:10,-10:0.1:10);
ineq = (0<=y) & (y<=3) & (9-y.^2<=x) & (x<=9) & (0<=z) & (z<=9-x);
But all that does is show a non-convex domain, represented by scattered data points that lie inside the domain. Surf is not designed to do what you want.