First of all, we must clarify a concept. The physical center of gravity refers to the equivalent center of mass, that is, the mass of the equivalent system is concentrated at one point. So this question is actually to ask which point three balls with a total mass of 4m can be equivalent to an object with a mass of 4m, so since they are a system, they can be equivalent in pairs, because the following two balls are both M, so it is convenient to see that the equivalent point is BC midpoint. Assuming that this point is D and the equivalent particle D has a mass of 2m, then the two points A are 2m. After a and d are equivalent, it is the center of gravity of the system, that is, the AD midpoint with a mass of 4m. Can you understand this? Ask again if you don't understand.