You are trying to solve the problem of minimizing
3*x + 3.4*y
subject to the constraint
3*x + 4*y <= 20000
with bounds of [0,10000] for both x and y.
The solution seems pretty clear to me. It will happen at x=y=0. What does linprog say?
linprog(f,A,b,,,[0 0],[10000 10000])
Optimal solution found.
So it had no problems at all.
My guess is one of two things happened. You might be using an old MATLAB release, and possibly there was a bug. I seriously doubt that. As this is a trivial problem to solve.
An alternative is you have your own code named linprog. So do these things. First, tell us what release you are using. If you don't, then all I can say is sorry, but it worked perfectly fine using a current release.
Then, look closely at the result of:
Does it show something strange? Have you written your own version of linprog? There should be only one copy of linprog. In my case, it lives here: