Answer
The speed of the two-body center of mass is $~~2.5~m/s$
Work Step by Step
Note that the speed of the center of mass does not change during a collision.
In part (a), we found that the mass of the other body is $1.2~kg$
We can use the initial conditions to find the speed of the two-body center of mass:
$v_{com} = \frac{(2.0~kg)(4.0~m/s)+(1.2~kg)(0)}{2.0~kg+1.2~kg}$
$v_{com} = 2.5~m/s$
The speed of the two-body center of mass is $~~2.5~m/s$
