In a uniform magnetic field of strength 0. 15T , a short bar magnet...

For the unstable equilibrium, the angle between the magnetic moment and magnetic field is 180^{o}. ( $∵$ In this position it will be in a direction perpendicular to magnetic field thus maximum torque will act on it.)

$\theta = 1 8 {0}^{o}$

Potential energy of the magnet

U = - mB \cos 180^{o}

= - 0.32 x 0.15 (- 1) = 4.8 x 10^{-2} J

Thus, for the unstable equilibrium the potential energy is 4.8 x 10^{-2} J.