Answer:

k=17.32 N/m
Step-by-step explanation:
Given
mass of cart A =500 gm
mass of cart B =1000 gm
Spring is compressed by a distance of 5.5 cm
B's recoil speed is 0.55 m/s
Conserving momentum


Thus
recoil speed is greater than B
Conserving Energy to get value of K


k=17.32 N/m