OK, so the downward force on the rock is mg (force of g) - 2.8N (force in opposite direction). since mg (9.6 kg*9.8 m/s^2)= 94.08 N, then mg-2.8= 91.28 N. That is your net force on the rock (or total force). And, since f=ma, a= f/m.
so, a= 91.28 N / 9.6 kg = 9.5 m/s^2.(final answer)
