Moments a level question

hi, so i was doing this question (attached), and how i went about it was equating the reaction force at x to 0, finding the force required to keep the system in equilibrium, and repeating with equating the reaction force at y to 0, and writing down the larger mass.

the answer says that the correct answer is determined by equating the reaction force at x specifically, however, and i was wondering whether anyone could explain as to why. thanks.