Please see the attachments.
Power= mom*ang vel. Please use work kinetic energy concepts.
By energy law,
Initial mechanical energy = final mechanical energy
= potential enrgy of spring (PESi)
+ gravitational potential energy of rod (PERi) (reference is B)
= potential energy of spring(PESf)
+ rotational kinetic energy of rod (RKERf)
Because, natural length of spring,
lo = 225 mm
initial length of spring,
li = ...
This solution uses energy law to find angular velocity. It's provided in 590 words and includes an attached .jpg image to enhance understanding.