Explanation

(d) If $n$ masses are equal
$m_{1} = m_{2} = m_{3} = .... = m_{n}$
Then position vector of centre of mass.
The coordinate of the centre of mass
$(x,y) = \left( \frac{m_{1}x_{1} + m_{2}x_{2} + m_{3}x_{3}}{m_{1} + m_{2} + m_{3}},\ \frac{m_{1}y_{1} + m_{2}y_{2} + m_{3}y_{3}}{m_{1} + m_{2} + m_{3}} \right)$
$= \left( \frac{m + 2m + 3m}{3m},\frac{m + 2m + 3m}{3m} \right)$
$= \left( \frac{6m}{3m},\frac{6m}{3m} \right)$
$= (2,2)$