Answer
The speed of the center of mass is $~~72~km/h$
Work Step by Step
We can find the speed of the center of mass:
$v_{com} = \frac{m_1~v_1+m_2~v_2}{m_1+m_2}$
$v_{com} = \frac{(2400~kg)(80~km/h)+(1600~kg)(60~km/h)}{2400~kg+1600~kg}$
$v_{com} = 72~km/h$
The speed of the center of mass is $~~72~km/h$
