Step 1: FBD - Draw a free body diagram of A+B+bar.
Step 2: Work/energy - Write down the individual KE and PE for A and B. Add together these to find the total KE and PE for the system.
Step 3: Kinematics - Consider the location of the instant center for link AB for the position of interest where AB is horizontal. What does this say about the velocity of B at that position?
Step 4: Solve - Combine your equations from Steps 2 and 3 to solve for the speeds of A and B.