Alternative solution

Model, which mathematical formulation is as follows: 

The intersection of feasible regions of both variables is unlimited. The iso-profit line can be shifted to the right with no limits. The optimal solution is unbounded. The only help in such a case is to add constrain, to find some limit or demand. This finding is very likely to be successful because no real process is without limit.

The feasible area for the first constrain was drawn using points A(0;1) and B(4;0) ,

for second constrain points C(0;2) and D(4;4), for third constrain E(0;6) and F(6:0)

The iso-profit line has the same direction as the line limiting the area of third constrains. When we move the iso-profit line to the boundary of the feasible area , the intersection is not a point but a line between the points F and G ( unlimited set of optimal solutions).