Locate the center of mass.

Explanation

(a) Take location of the leftmost particle as origin.
`{:(,x),(m,0),(2m ,L),(3m ,L + 2L=3L):}`
`x_(c.m.) = (m_(1) xx 0 + m_(2) xx d)/(m_(1) + m_(2)) = (m_(2) d)/(m_(1) + m_(2))`
( c) Take the bottommost particle as origin.
`{:(,y),(m , 0),(2m , L),(3m , L + 2L = 3L):}`
`y_(c.m.) = ( m xx 0 + 2m xx L + 3m xx 3L)/( m + 2m + 3m) = (11 mL)/(6 m) = (11 L)/(6)`