For the unstable equilibrium, the angle between the magnetic moment and magnetic field is 180^o. ( $\because$  In this position it will be in a direction to magnetic field thus no torque will act on it.)

       $\theta =180^{\text{o}}$

The potential energy of the magnet

      $\mathrm{U}=-\mathrm{mB} \cos {180}^{°}$

            $=- 0.32\times 0.15 (- 1) = 4.8\times {10}^{-2} \mathrm{J}$

Thus, for the unstable equilibrium, the potential energy is $4.8\times {10}^{-2} \mathrm{J}$.