Ared ball with a velocity of 3.0 m/s collides head-on with a yellow ball of equal mass moving with a velocity of ? 2.0 m/s. what is the velocity of the two balls after the elastic collision?

The center of mass of the two objects is moving with a velocity of 2.5 m/s. Since there are no external forces, that velocity is not going to change.
Before the collision, the red ball approaches the center of mass with a velocity of +0.5 m/s and the yellow ball with a velocity of -0.5 m/s.
After the collision, the velocities have the same magnitude and opposite directions, seen from the center of mass (energy and momentum must be conserved).
If the red ball moves away from the center of mass with a velocity of -0.5m/s, its velocity in the normal frame becomes 2 m/s. 
And if the yellow ball moves away from the center of mass with a velocity of +0.5m/s, its velocity in the normal frame becomes 3 m/s.