please help Asap commutator practise chemical Engineering

Evaluate the commutator (P2, [Pz, H]], where pz is the z-component of the momentum operator and H = m +V() is the 2 r Hamiltonian of the system. 2m Use this result to prove the following sum rule: (En Em)|(n|pz|m)| ( n2 = A a2v az? 2 n where n) and (m) are eigenvectors of H with eigenvalues En and Em, respectively