Answer:
The speed of center of mass is 3.64 m/s.
Explanation:
Given that,
Total mass of the group of particles, m = 35 kg
Total kinetic energy of group of particles, 
The kinetic energy relative to the center of mass is, K = 81 J
We need to find the speed of center of mass. We know that the total kinetic energy is equal to the sum of rotational kinetic energy and the rotational kinetic energy.

Since,
,
is the speed of center of mass

So, the speed of center of mass is 3.64 m/s.