Dear Student,
As we know the torque on a dipole placed in uniform electric field is given by,
ζ = p X E = pE sinθā = pEsin60āo = [given in the ques, angle = 60o]
the value of tourque is given in ques, ζ = Nm
so we get, pE = 12 ................(1)
Now we know potential energy of dipole, U = - p . E =- pE cosθā = -pE cos60o= - pE / 2 ..................(2)
Putting the value of 'pE' from eqn (1) to eqn (2) we get,
U = - 12 / 2
U = - 6 J
Regards.