**Answer:**

The system's kinetic energy changes by 3.6 J

**Explanation:**

The given parameters are;

The number of cart = 2

The mass of each cart = 0.5kg

The initial length of the spring = 0.50 m

The final length of the spring =T0.3 m

The change in position of the first cart = 0.6 m

The energy given to the first cart = Work done by the force = Force × Displacement

The initial kinetic energy of the two cart moving together = Energy given to the first cart = 6.0 × 0.2 = 1.2J

The kinetic energy given to the two cart combined = The applied force × The total displacement of the two cart as they move together

The kinetic energy given to the two cart combined = 6.0 × (0.6 - 0.2)

The kinetic energy given to the two cart combined = 6.0 × 0.4 = 2.4 J

The total kinetic energy given to the two cart = 1.2 + 2.4 = 3.6 J

The total kinetic energy given to the two cart = 3.6 J

The system's kinetic energy changes by 3.6 J.