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from where you got 0.75 for x3? as i see in the book, we have only 0.50m, 0.25m and nothing else! so can you explain that please?

Hey milad90, thanks for the comment. The left ball was chosen as the "origin", which means all the measurements are made with respect to it. This means the right hand ball is a total of (0.50m plus 0.25m) = 0.75m from the "origin". You always need to choose a reference point (an "origin") for making these center of mass calculations. Hope this helps!