ok so let position of the CM be R and let's work in the x y z coordinate system
Then R=[m(0,0,0)+m(2a,0,0)+m(4a,0,0)+3m(2a,2a,0)]/6m
which simplifies to
R=((4/3)a,(1/3)a,0)
Then using the equation Impulse=(mtotal)(vcm) we can write:
P=(6m)(vcm)
vcm=p/6m
Now to find the initial angular...