Do you see that blank, open space after the word "potential ..." ?
There's supposed to be a number there that actually tells us the value of the potential. Without that number ... and a lot more description of the whole scenario here ... there's no possible answer to the question.
Answer:
Explanation:
Given
Both cars mass is m
and solving problem in Vertical and horizontal direction
considering + y and +x to be positive and u be the final velocity of system
Conserving Momentum in Vertical direction

------1
Conserving momentum in x direction
-----2
squaring and adding 1 &2




It takes twice the force to produce the same acceleration in the 2kg rock.