Given:
The mass of the truck is m1 = 3162 kg
The speed of the truck is v1i = 12 m/s in East
The mass of the parked car is m2 = 510 kg
The speed of car is v2i = 0 m/s
The speed of car after collision is v2f = 24 m/s in East
To find the speed of the truck after collision.
Explanation:
The final velocity of the truck will be

Thus, the speed of the truck after collision is 8.129 m/s
Explanation:
Given
initial speed(u)=3 m/s
mass of each ball is m
Let the cue ball is moving in x direction initially
In elastic collision Energy and momentum is conserved
Let u be the initial velocity and
be the final velocity of 8 ball and cue ball respectively

The angle after which cue ball is deflected is given by

Conserving momentum in x direction


Along Y axis


substitute the value of 
we get 

Answer:
The average velocity from 5 to 6 seconds is -27 m/s