How would an object’s kinetic energy change if it’s mass decreased by a factor of 3 and it moved 6 times faster?

Decreases in mass cause decreases in kinetic energy due to the aforementioned positive relationship between the two. Therefore when mass is decreased, an increase in kinetic energy is possible if velocity increases.

what was the acceleration of the 2 kg cart?

⇒ 16.0 cm/s2

what was the acceleration of the 4 kg cart?

⇒ 8.1 cm/s2

what was the acceleration of the 6 kg cart?

⇒ 5.3 cm/s2