Arock is rolling down a hill. at position 1, its velocity is 2.0 m/s. twelve seconds later, as it passes position 2, its velocity is 44.0 m/s. what is the acceleration of the rock?

As this happens over twelve seconds, you would take the total difference in velocities and divide it by twelve to find the change per second

44.0 m/s - 2.0 m/s = 42.0 m/s 

42.0 m/s / 12 s = 3.5 m/s2

the acceleration of the rock would be 3.5 m/s2