A2 kilogram rabbit starts from rest and is moving at 6m/s after 3 seconds. what net force must be exerted on the rabbit (by the ground) to cause this change in speed?

The best way to handle this is to apply Newton's
second law of motion:

       Force = (mass) x (acceleration) .

But before we can do that, we need to figure out
the rabbit's acceleration.

       Acceleration  =  (change in speed) / (time for the change)

                                             =  (6 m/s)  /  (3 sec) 

                                             =  2 m/s² .

Now we have both of the numbers we need in order to apply Newtons law:

           Force = (mass) x (acceleration)

                     =   (2 kg) x (2 m/s²)

                     =        4 kg-m/s²      =    4 newtons .