maximum extension in the spring

Question

two blocks A(3kg) and B(6kg) are connected by a spring of stiffness 512n/m and placed on a smooth horizontal surface. Initially has it's equilibrium length. Velocities 1.8m/s and 2.2m/s are imparted to A and B in opposite direction. The maximum extension in the spring will be-

Answer 1

At maximum extension of the spring, velocities of both blocks will be same.   

Conservation of momentum: 

6*2.2 – 3*1.8 = (3+6)*V 

V = 0.867 m/s   

Conservation of energy: 

1/2*3*1.8*1.8 + 1/2*6*2.2*2.2 

= 1/2*(6+3)*0.867*0.867 + 1/2*512* X*X 

X = 0.25 m = 25 cm