Can a resistance be negative?
Notice that you have a current source and not a voltage source. In this case, when using Thevenin you should replace the current source by an open circuit to find the equivalent resistance (as opposed to replace by a short if you have a voltage source). Therefore, the equivalent resistance will not be R1//R3 + R2 as you've assumed. Actually the solution is much simpler (I'll let you finish it).

Let's suppose that you did everything right and ended up with a negative resistance. That would simply tell you that there is no solution to the problem (what is not the case here).