The ball takes 1.5s to go from B to C and 0.3s to go from A to B. By symmetry, the ball will take 0.3 seconds to go from C to D, so the total time of flight is 0.3s+1.5s+0.3s = 2.1s Also by symmetry we know that the final velocity is equal to the negative of the initial velocity.
Or an alternative solution utilizes an overall displacement of zero during the whole flight:
This is equal to what we had above and we get the same answer.