It looks to me as though, it is a trick question. The V2 supply has the + positive lead hooked to R2. If their is no current flow Vo should be +6 volts.

It looks to me as though, it is a trick question. The V2 supply has the + positive lead hooked to R2. If their is no current flow Vo should be +6 volts. Even if V2 had the negative lead hooked to R2 the diode would be reverse bias and not conduct. Still +6 volts.