Discussion and hints:

For this problem using the linear impulse-momentum equation, work with the system of A+B+C as a whole when looking for motion. If you then need to find internal forces to the system, consider individual FBDs for the different bodies.

Step 1: FBDs
For Part (a), work with the system of A+B+C as a whole. For Part (b), consider individual FBDs for A, B and C.
Step 2: Kinetics (linear impulse/momentum)
Look for forces acting in the x-direction for each of your FBDs. When is linear momentum conserved? And, when linear momentum is not conserved, how can you find the average values of the forces acting on the FBD?
Step 3: Kinematics
Step 4: Solve

---

Any questions?