I am solving from the equation torque=-(PE)sin@. say (theta = @)
also we know that torque=moment of inertia * angular acceleration.
Ok. so we get
I*a=-(PE)sin@. (here a= angular acceleration.)
since @ is very small so sin@=@.
a=-(PE)/I @ -----1
from equation of SHM,
compare equation 1 and 2
we get w=sqrt(PE/I)
and T=2 pi/w;
so T= 2pi sqrt(I/PE).