Answer:
av=0.333m/s, U=3.3466J
b.

Explanation:
a. let
be the mass of block A, and
be the mass of block B. The initial velocity of A,
-The initial momentum =Final momentum since there's no external net forces.

Relative velocity before and after collision have the same magnitude but opposite direction (for elastic collisions):

-Applying the conservation of momentum. The blocks have the same velocity after collision:

#Total Mechanical energy before and after the elastic collision is equal:

Hence, the maxumim energy stored is U=3.3466J, and the velocity=0.333m/s
b. Taking the end collision:
From a above, 
We plug these values in the equation:

