Answer:
c.m=(1/∑mi)*∑mi*yi

Step-by-step explanation:
We solve this problem, with the equation for the center of mass:
c.m=(1/∑mi)*∑mi*yi
represent the different mass of the system
represent the position of the different mass of the system
c.m=(1/∑mi)*∑mi*yi=(1/(m+2m))*(m*d+2m*3d)=7d/3