Please help I$\text{'}$m studying old exams for the final and need help understanding this problem

For the system shown on the right, blocks A and $B$ have masses $m_A$ and $m_B,$ respectively. Pulley $C$ has radius $R_C,$ mass $m_C,$ and mass$-$moment of inertia $I_C.$ Pulley $D$ has radius $R_D$ and inertia $I_D.$

a$.[10$ pts$]$ Calculate the equivalent mass with respect to $x_B,$ the motion of block $B.$

b$.[10$ pts$]$ Draw an FBD of the equivalent system and develop the equation of motion in terms of $x_B.$ You are not required to substitute your answer for $m_e$ from part $($a$)$ into the EOM.