Answer:
a) 
b) 
Explanation:
Given:
mass of the lighter block, 
velocity of the lighter block, 
mass of the heavier block, 
velocity of the heavier block, 
a)
Using conservation of linear momentum:

where:
final velocity of the lighter block
final velocity of the heavier block

........................(1)
Since kinetic energy is conserved in elastic collision:



divide the above equation by eq. (1)
.............................(2)
now we substitute the value of v from eq. (2) in eq. (1)



(negative sign denotes that the direction is towards left)
b)
now we substitute the value of v' from eq. (2) in eq. (1)



